Trace Elements in Sediments of the Lower Eastern Coast of the Firth ...

Environment Waikato Technical Report 2007/08 

Trace Elements in 

Sediments of the Lower 

Eastern Coast of the 

Firth of Thames 

www.ew.govt.nz 

ISSN 1172-4005 (Print) 

ISSN 1172-9284 (Online)

Prepared by: 

Nick Kim, based on a report from URS New Zealand Ltd. 

For: 

Environment Waikato 

PO Box 4010 

HAMILTON EAST 

May 2007 

Document #: 1120743

Peer reviewed by: 

Matthew Taylor Date May 2007 

Approved for release by: 

Peter Singleton Date June 2007 

Disclaimer 

This technical report has been prepared for the use of Waikato Regional Council as a reference 

document and as such does not constitute Council’s policy. 

Council requests that if excerpts or inferences are drawn from this document for further use by 

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Doc # 1120743

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Executive summary 

Background 

In 2004, Environment Waikato tested for trace elements in shallow sediments from five 

sites spread across the lower Firth of Thames. Results indicated the presence of 

moderately elevated mercury in sediments at three of the sampling locations and 

suggested a need for further investigation. Environment Waikato then commissioned 

URS New Zealand Ltd (URS) to undertake more extensive sediment sampling of the 

eastern coast of the lower Firth of Thames and to produce a report on this. During June 

2005, URS collected 78 sediment samples from the following 11 locations: Kuranui 

Bay, Piako River mouth, Tararu, Tapu, Te Mata, Te Puru, Thames mudflats, Thames 

urban area, Thornton Bay, Waihou River mouth, and Waiomu. All sediment samples 

were analysed for arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. 

Samples collected from selected sites also underwent analysis for grain size, lithium, 

iron and aluminium, to aid in interpreting the results. Statistical tests were applied to 

ascertain if there were significant differences between sites. URS submitted their final 

report and assessment to Environment Waikato in 2006. This review has been adapted 

from the URS report and incorporates additional data and information which has come 

to light since sampling was carried out. 

Findings: relative enrichments 

Arsenic, cadmium, copper, mercury, lead and zinc are enriched in sediments of the 

lower eastern coast of Firth of Thames, relative to concentrations present before 

Polynesian and European colonisation, and reference concentrations in sediments 

from Raglan Harbour. Concentrations of the other five elements measured in some or 

all samples (chromium, nickel, aluminium, iron and lithium) are more typical of those 

observed in harbour sediments in other areas. Relative to its expected background 

concentration, the most highly enriched element is mercury. Mercury concentrations 

are on average about seven times higher in Firth of Thames sediments than those of 

reference sites. In mass terms, the most highly enriched element is zinc. Firth of 

Thames sediments contain about 10 mg/kg more zinc than reference sites. 

Findings: local and general sources 

Only one area (at two adjacent sampling locations) stands out as a hotspot of localised 

metal contamination, which looks likely to have been from an urban industrial source. 

This is the sediment in Kuranui Bay and south of this in the area of the Thames 

pipeline. Results from depth profile samples suggest that the worst contamination in 

this area may have been historic. Subsequent information suggests an association with 

industrial fill or landfill as a part of historic land reclamation in the area - the Moanataiari 

reclamation. Risks associated with this contamination are to organisms living in nearby 

marine sediments. There is currently little evidence of risk to people living on the 

reclamation. 

Apart from this area, the influence of local sources appears minor. The results provide 

little evidence that former mining sites along the lower eastern coast of the Firth of 

Thames had, or continue to have, a significant impact on the quality of their nearest 

coastal sediments, relative to the more general impact from other sources. 

Although there is some evidence for the presence of the occasional grain of a metalrich 

mineral in sediments, more generally the data suggests that elevated arsenic, 

copper, lead, cadmium and zinc concentrations along the lower eastern coast of the 

Firth of Thames are likely to be dominated by larger-scale sources, which have been 

capable of causing an impact over the area as a whole. The three most likely largescale 

sources are enhanced weathering and erosion following land clearance (arsenic 

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and copper), the impact of distant historic mining operations which involved disposal of 

large volumes of tailings directly to the Ohinemuri River (zinc, cadmium, and lead) and 

agricultural inputs (zinc and cadmium). A fourth possible source identified in this work 

(for mercury) is dissolved organic matter entering the Firth of Thames from wetlands 

and peatlands of the Hauraki Plains. Further work is needed to confirm this hypothesis. 

Findings: comparison to guidelines 

Although concentrations of copper, cadmium, lead and zinc were higher than typical 

values for uncontaminated sediments, they are still well below the lowest sediment 

quality guideline values, and are believed to pose a low level of risk to health of aquatic 

ecosystems. The two elements that are nearest to or occasionally exceed guideline 

values for sediments are arsenic and mercury. Arsenic concentrations in coastal 

sediments at some locations in the Firth of Thames are at a point where they may 

adversely affect some sediment-dwelling organisms. Sediments in the area of the 

Moanataiari reclamation are substantially enriched in mercury, and consistently exceed 

relevant sediment quality guideline values. In URS NZ Ltd’s sampling round, arsenic 

exceeded its ISQG-Low guideline (20 mg/kg) at eight out of nine (89% of) sites. 

Mercury exceeded its ANZECC (2000) ISQG-Low guideline (0.15 mg/kg) at four out of 

nine (44% of) sites. 

Findings: most likely sources where enriched 

Lead 

Results for lead are consistent with the existence of one significant diffuse source of 

additional lead to the Firth of Thames, which is most likely to have been the influence 

of past mining in the Ohinemuri catchment. The Waihou River is known to have 

received between 500-800 tonnes per annum of mine tailings from the Ohinemuri River 

for over 50 years. This source is also likely to have contributed substantial zinc and 

cadmium (see below). 

Zinc and cadmium 

Results for zinc and cadmium are most consistent with the existence of two significant 

diffuse sources to the Firth of Thames: past mining, and current agricultural treatments. 

The Ohinemuri mining source (see lead above) may have been a significant source of 

three metals: lead, zinc and cadmium. Agriculture may be associated with two metals: 

zinc and cadmium. The most likely agricultural source of zinc is zinc sulphate, which is 

used in large volumes as a remedy for facial eczema in stock. The most likely 

agricultural source of cadmium is phosphate fertilisers (primarily superphosphate), in 

which cadmium is present as an impurity. 

Arsenic and copper 

Results for arsenic and copper are consistent with one primary diffuse source to the 

Firth of Thames: weathering of minerals such as pyrite in coastal areas of the 

Coromandel Peninsula. It is likely that the rate and extent of weathering and erosion, 

and influx of arsenic and copper, was enhanced by historic land clearance activities 

around the coastal areas of the Coromandel Peninsula, following Polynesian and 

European settlement of the area. 

Mercury 

There is no positive evidence that past mining, erosion of natural minerals, or 

agricultural treatments have been significant sources of mercury to the Firth of Thames 

sediments. Rather, the results suggest a source of mercury which is delivered to the 

Firth of Thames via the Piako River. Based on recent literature, it is thought that 

significant inputs of mercury may originate from drainage of the wetlands and 

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peatlands of the Hauraki Plains. This form of mercury is most likely to be associated 

with dissolved organic carbon in the drainage waters. In addition, the base of the 

Moanataiari reclamation in Thames appears to be a localised hotspot of mercury 

contamination to marine sediments. 

Recommendations 

The following recommendations are made. (For further details of each 

recommendation, and its rationale, see Section 4.2.) 

1. Sediments of the Firth of Thames be sampled once every five years, to allow early 

warning in the event that concentrations of one or more trace elements are 

gradually increasing. 

2. Further work on mercury is commissioned, with a focus on: 

• Quantifying the relative significance of wetlands and peat deposits in the 

Hauraki Plains as sources of mercury to the Firth of Thames, relative to other 

sources. 

• Identifying land management factors that would increase or decrease mercury 

inputs to the Firth of Thames. 

• Identifying the most significant risks associated with mercury which has entered 

the Firth of Thames sediment reservoir, focusing on the significance of mercury 

to wildlife and human food sources, and the impact of mangrove colonisation on 

mercury chemistry. 

3. The feasibility of simple erosion control measures that might work to reduce the flux 

of arsenic entering the Firth of Thames be investigated. 

4. Contaminated site investigations be carried out on sediments in the area of the 

Moanataiari reclamation to determine hazards to the ecosystem, pathways, risks to 

the marine ecosystem, and management options. 

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Note on terms 

Heavy metals / trace elements 

The focus of this report is on concentrations and sources of eight chemical elements: 

arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), 

lead (Pb) and zinc (Zn). These elements are often referred to as ‘heavy metals.’ 

However, this term is falling out of favour because it is an ambiguous one. A range of 

different definitions for ‘heavy metal’ exist in the scientific literature. In addition, arsenic 

is not regarded as a true metal, but a metalloid. The term ‘trace element’ is used in this 

report because it is not ambiguous, and accurately describes the group of eight 

elements that are the focus of this work. A ‘trace element’ is something which is not 

one of the ten major elements. Ninety-nine percent of the earth’s crust is composed of 

the ten major elements: silicon, oxygen, aluminium, iron, calcium, potassium, sodium, 

magnesium, titanium and phosphorus. All other elements are ‘trace elements’, and 

most are present at natural concentrations of well under 100 mg/kg (parts per million) 

in the earth’s crust. The term ‘metal’ is sometimes used in this report in discussing an 

element other than arsenic. 

Enrichment / contamination 

Trace elements occur naturally. When their concentrations are higher than expected, 

they are usually referred to as being ‘enriched,’ or ‘elevated’ above their natural 

concentrations. The terms ‘contaminated’ or ‘contamination’ are usually reserved for 

cases where a trace element’s concentrations have become sufficiently high to cause 

significant adverse effects on the environment. For convenience, this is usually 

assessed by reference to sediment quality guidelines. Sites would normally be 

regarded as contaminated when trace elements are present at concentrations that 

significantly exceed the ANZECC ISQG-High (see Section 2.5). These conventions are 

followed in this report. 

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Table of contents 

Executive summary i 

1 Introduction 1 

1.1 Potential sources of the elements 1 

1.1.1 Weathering and erosion 1 

1.1.2 Past mining 3 

1.1.3 Agricultural and horticultural inputs 4 

1.1.4 Urban inputs 5 

1.2 Previous data 5 

2 Study methodology 9 

2.1 Sampling objectives 9 

2.2 Sampling locations 9 

2.2.1 General 9 

2.2.2 History of the sample locations 15 

2.3 Sample types 16 

2.3.1 Manual grab samples 16 

2.3.2 Vertical profiles 16 

2.3.3 Grab sampling 17 

2.4 Analysis and analytical quality assurance 17 

2.5 Use of ANZECC sediment quality guidelines 18 

3 Results and discussion 19 

3.1 Raw results and summary statistics 19 

3.2 Comparison with sediment quality guidelines 20 

3.2.1 Comparison to the ISQG-Low 20 

3.2.2 Comparison to the ISQG-High 21 

3.2.3 Key findings 21 

3.3 Comparison between the Firth of Thames and other areas 22 

3.3.1 Comparison to coastal sediments in Raglan Harbour 22 

3.3.2 Comparison of this survey’s data with historical data 24 


3.4 Small scale variability 25 

3.4.1 Assessment 25 

3.4.2 Key finding 26 

3.5 Comparisons between sites in the Firth of Thames 26 

3.5.1 Statistical analysis and data normalisation 26 

3.5.2 Local historical mining sites versus control sites 27 

3.5.3 Seaward versus landward 30 

3.5.4 Urban areas versus control sites 30 

3.5.5 Agriculturally proximate versus control sites 34 


3.6 Correlations and spatial trends 36 

3.6.1 Approach and correlation matrix 36 

3.6.2 Zinc, cadmium, and lead 37 

3.6.3 Arsenic and copper 39 

3.6.4 Chromium and nickel 41 

3.6.5 Mercury 41 

3.6.6 Principal Components Analysis 44 


4 Summary and recommendations 47 

4.1 Summary 47 

4.1.1 Relative enrichments 47 

4.1.2 Comparison to guidelines 47 

4.1.3 Local and general sources 47 

4.1.4 Probable dominant sources by element 48 

4.2 Recommendations 49 

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References 51 

Appendix 1. Further details of each location from which sediment samples were 

collected by URS as part of this study. 53 

Appendix 2. Measured concentrations of eight trace elements in 78 sediment 

samples collected as part of this study. 56 

Appendix 3. Quality assurance and quality control results 59 

Appendix 4. Metal concentrations in sediments normalised against various 

benchmark variables. 61 

Appendix 5. Statistical summary of results for eight trace elements in each of 

the ten areas sampled. 65 

Appendix 6. Concentrations of 33 elements, total organic carbon and dry matter in 

composite shallow (0-2 cm) sediment samples collected in October 2003 from five 

locations in the Firth of Thames and five locations in Raglan Harbour. 70 

Appendix 7. Results of Student’s t-tests between Thames and Kuranui Bay 

sites and three mining reference sites (Te Puru, Thornton and Te Mata). 71 

Appendix 8. Map showing the full extent of catchments feeding the lower Firth 

of Thames. 72 

List of figures 

Figure 2-1: Context of the study area, showing catchment boundaries and other 

key features. 10 

Figure 2-2 Locations of sites where sediment was sampled in 2005 and old 

mining sites. Exact coordinates of the sediment sampling sites, and 

details about the numbers of samples collected, are provided in Table 

2-1. 14 

Figure 2-3 Diagram indicating approach to collection of composite samples 

obtained from stream mouths. 16 

Figure 2-4 Diagram indicating approach to collection of composite samples 

obtained from intertidal zone with no significant inputs. 17 

Figure 3-1 Apparent enrichment of cadmium relative to the three mining control 

sites moving from south to north. 29 

Figure 3-2 Aerial photograph of the Moanataiari reclamation dated 24 June 

2006. Imagery sourced from Terralink International Ltd (TIL) 2006 

and is the property of TIL and the Waikato Regional Aerial 

Photography Service (WRAPS) 2006. Copyright Reserved. 32 

Figure 3-3 Relationship between zinc and cadmium concentrations (mg/kg) in 

Firth of Thames sediment samples. (See Section 3.6.1 for information 

about data set coverage.) 38 

Figure 3-4 Relationship between concentrations of zinc in sediments and 

distance from the Waihou River mouth in kilometres. (See Section 

3.6.1 for information about data set coverage.) 38 

Figure 3-5 Relationship between arsenic and copper concentrations (mg/kg) in 

Firth of Thames sediment samples. (See Section 3.6.1 for information 

about data set coverage.) 39 

Figure 3-6 Average concentrations of mercury moving from the Piako River 

mouth east and then north, excluding the Thames pipeline/Kuranui 

Bay anomaly. (Other details are as for Table 3-15.) 42 

Figure 3-7 Relationship between concentrations of mercury and arsenic, and 

mercury and copper, in Firth of Thames sediments. Site coverage is 

as described in Table 3.15. 44 

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List of tables 

Table 1-1 Environmentally significant metals which occur at elevated 

concentrations in some New Zealand rocks (adapted from Metals in 

New Zealand Environment, University of Otago). 2 

Table 1-2 Alteration, mineralisation and old mine workings in the Coromandel 

area (modified from Livingston 1987, including data from Jenkins 

1991 and Moore et al. 1996). 4 

Table 1-3 Concentrations of selected elements in phosphate fertiliser, soils and 

rocks. 5 

Table 1-4 Contaminants in New Zealand stormwater (from Taylor et al. 2005). 5 

Table 1-5 Sediment quality data (mg/kg) obtained by other authors prior to this 

study. (Refer to Table 1-1 for the element name associated with each 

chemical symbol.) 7 

Table 2-1 Details of sediment samples collected by URS New Zealand Ltd in 

2005. 11 

Table 3-1 Summary of trace element concentrations at sampling sites and 

comparison to ANZECC (2000) sediment quality guidelines (all values 

in mg/kg dry weight). Site locations are listed from south (Waihou and 

Piako River mouths) to north (Te Mata). Bold entries exceed 

ANZECC (2000) ISQG-Low values; the bold italic entry exceeds an 

ISQG-High value. Refer to Table 1-1 for the element name 

associated with each chemical symbol. 19 

Table 3-2 Summary results expressed as a fraction of the lowest ANZECC 

(2000) sediment quality guideline (the ISQG-Low). Refer to Table 1-1 

for the element name associated with each chemical symbol. 20 

Table 3-3 Summary results expressed as a fraction of the upper ANZECC 

(2000) sediment quality guideline (the ISQG-High). 21 

Table 3-4 Average concentrations of eleven elements in shallow surface (0-2 

cm) sediments collected from Raglan Harbour and the Firth of 

Thames in earlier work, and comparison of averages between the two 

areas. 23 

Table 3-5 Small scale variability in chemical composition (mg/kg). (%RSD 

stands for per cent relative standard deviation, also known as the 

Coefficient of Variation). 25 

Table 3-6 Results of Student’s t-tests between Tararu, Waiomu, and Tapu and 

pooled data from the three mining control sites. Mean values are in 

mg/kg (dry weight), and have not been normalised to any other 

variable. Ratios of means are unitless. 27 

Table 3-7 Apparent enrichment of cadmium and zinc in sediments relative to the 

three mining control sites moving from south to north. Numbers are 

enrichment ratios, representing the average concentration in 

sediments at a given site divided by the pooled average for the 

mining control sites. 29 

Table 3-8 Results of Student’s t-tests between Thames and Kuranui Bay sites 

and other coastal sites. Mean values are in mg/kg (dry weight), and 

have not been normalised to any other variable. Ratios of means are 

unitless. 31 

Table 3-9 Trace element concentrations in shallow (0-2 cm) sediment samples 

from Kuranui Bay which are statistically higher than those at four 

other sites in the Firth of Thames at a 95% confidence level. 33 

Table 3-10 Concentrations of mercury in 0-14 cm sediment samples collected 

from the vicinity of the Moanataiari reclamation, and comparison to 

ANZECC (2000) guideline values for sediment quality. Data source: 

Kingett Mitchell Ltd, 2004. 34 

Table 3-11 Results of Student’s t-tests between sites closest to agricultural 

sources and other areas. Mean values are in mg/kg (dry weight), and 

have not been normalised to any other variable. 35 

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Table 3-12 Pearson’s correlation coefficients for relationships between element 

concentrations in sediments (N = 62 pairs). Yellow shaded boxes 

represent highly significant relationships with probability values of 

p0.474) and pR>0.408). (See text for 

information about data set coverage.) 36 

Table 3-13 Pearson’s correlation coefficients between element concentrations at 

the coastal sites and distance from the Waihou River mouth (N = 49 

pairs). Yellow shaded boxes represent highly significant relationships 

with probability values of p0.527) and pR>0.456). (See text for information about data set coverage.) 37 

Table 3-14 Pearson’s correlation coefficients between element concentration and 

percentage of fines in the sediment (N = 15 pairs). Yellow shaded 

boxes represent highly significant relationships with probability values 

of p0.760) and pR>0.641). 37 

Table 3-15 Average mercury concentrations in surface (0-10 cm or grab) 

sediment samples from all sites. Sites in the vicinity of the Thames 

pipeline and Kuranui Bay are separated out due to evidence of a 

localised source of mercury, cadmium and zinc in this area (Section 

3.5.4). One outlier of 1.52 mg/kg mercury for Thames mudflat sample 

SDB584 has been removed from the data set prior to calculating 

these averages. 42 

Table 3-16 Results of Principal Components Analysis on the correlation matrix 

data set. Weightings less than -0.4 or greater than 0.4 are 

highlighted. 45 

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1 Introduction 

In 2004, Environment Waikato tested for trace elements in shallow sediments from five 

sites spread across the lower Firth of Thames. Results indicated the presence of 

moderately elevated mercury in sediments at three of the sampling locations, and 

suggested a need for further investigation. URS New Zealand Ltd (URS) was 

commissioned by Environment Waikato to undertake more extensive sediment 

sampling of the eastern coast of the lower Firth of Thames. In the expanded 

programme, samples were collected from sites stretching from the Piako River mouth 

eastward and then north, as far as Te Mata. The programme aimed partly to 

investigate whether elevated metal concentrations in surface sediments could be 

attributed to run-off from natural sulphide mineralisation, run-off from former mining 

operations/tailings dams, longer range transport of contaminants from the Waihou 

River (primarily from agricultural run-off or past mining) or from urban sources (e.g. 

stormwater or sewer pipelines from Thames). URS undertook the sampling between 22 

and 24 June 2005. 

Concentrations of eight elements were measured in samples collected by URS as part 

of this study: arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), 

nickel (Ni), lead (Pb) and zinc (Zn). 

This report presents the methodology and results of the sampling and analysis 

programme undertaken by URS, and combines this with previous data and new 

information to give an overall assessment of (a) trace element enrichments in 

sediments of the lower Firth of Thames, and (b) the most likely sources in cases where 

element concentrations are elevated. 

1.1 Potential sources of the elements 

At the outset of this work, it was recognised that trace element enrichment in Firth of 

Thames sediment may come about through four main sources: the influence of natural 

mineralisation, tailings from former mining operations, agricultural activities, and urban 

inputs. These possible sources are discussed below. As an outcome of this work, a fifth 

possible source has been identified. This is discussed in Section 3.6.5. 

1.1.1 Weathering and erosion 

• Natural weathering and erosion. Trace elements occur naturally in rocks and 

minerals in the earth’s crust and soils and sediments derived from these. Key 

primary minerals for 16 trace elements are shown in Table 1-1. The Coromandel 

range contains a significant natural abundance of volcanic hydrothermal minerals, 

including a number of notable ore deposits that have been mined commercially. 

Release of trace elements into surface run-off and streams which enter the Firth of 

Thames can come about through natural weathering and erosion processes. 

• Enhanced weathering and erosion. Enhanced loadings of trace elements are likely 

to have come about as a result of historic land clearance activities around the 

coastal areas of the Coromandel Peninsula, following Polynesian and European 

settlement in the area. Grazed hill country yields two to five times more sediment 

than similar terrain under native bush (Ritchie, 2000), while production forestry 

practices at planting and harvest remove groundcover and expose topsoil to 

erosion by overland flow (Basher, 2003). 

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Table 1-1 Environmentally significant metals which occur at elevated concentrations in 

some New Zealand rocks (adapted from Metals in New Zealand Environment, 

University of Otago). 

Metal Principal primary 

minerals 

Aluminium, Al Most rock-forming 


Geological setting Environmental 

mobility 

All settings High at low pH 

(AMD) 

Antimony, Sb Stibnite, Sb2S3 Schist-hydrothermal High 

Arsenic, As Arsenopyrite, 

FeAsS; 

pyrite, FeS2 (trace 

As); tetrahydrite 

other sulphide 


Cadmium, Cd Sphalerite ZnS 

(trace Cd) 

Chromium, Cr Chromite, FeCr2O4; 

Fe-Mg minerals 

Copper, Cu Chalcopyrite, 

CuFeS2 

Iron, Fe Many rock-forming 


Schist–hydrothermal; 

volcanic-hydrothermal; 

sulphurous coals; 

trace in most rocks 

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High 

Volcanic–hydrothermal High at low pH 

(AMD) 

Gabbro; ultramafic rocks; trace in 

most rocks 

Other sulphide minerals. Volcanic– 

hydrothermal; schist (rare) 

Moderate at 

low pH (AMD) 

High at low pH 

(AMD) 


(AMD) 

Lead, Pb Galena, PbS Volcanic-hydrothermal High at low pH 

(AMD) 

Manganese, 

Mn 

Many rock-forming 

minerals (with Fe) 

Mercury, Hg Cinnabar, HgS; 

some pyrite (trace 

Hg); some silver 

Molybdenum, 

Mo 


(AMD) 

Volcanic-hydrothermal Moderate-low, 

volatile 

Molybdenite, MoS2 Volcanic-hydrothermal Low 

Nickel, Ni Many rock-forming 

minerals (with Fe, 

Mg) 

All settings Moderate at 

low pH, 

especially coal 

AMD 

Tin, Sn Cassiterite, SnO2 Volcanic-hydrothermal Low 

Tungsten, W Scheelite, CaWO4 Schist-hydrothermal Low 

Uranium, U Uraninite, UO2 Rare, coaly sediments, Paparoa 

Range, Westland 

Zinc, Zn Sphalerite, ZnS Volcanic-hydrothermal; trace in most 

rocks 

High at high pH 

High at low pH 

(AMD) 

Once released, trace elements can be transported in surface or ground waters for 

considerable distances from the mine. Transportation is dependent on speciation, pH, 

redox conditions, precipitation of iron oxides, and the concentrations of suspended 

solids and dissolved organic matter within the water column. Trace elements may 

precipitate out and accumulate into sediments in areas where there is a sudden 

change of redox conditions, salinity or pH. Such changes are encountered where fresh 

water mixes with seawater. In addition, trace elements associated with colloidal

material and suspended particulate matter may settle out as sediment as water 

velocities slow. Lakes and estuaries provide suitable conditions for sediment 

accumulation that may have elevated trace element concentrations. 

1.1.2 Past mining 

The Coromandel Peninsula was the site of the first discovery of gold in New Zealand, 

at Driving Creek in 1852. From 1894 to 1910, low-grade quartz-vein deposits were 

actively worked using the cyanide extraction process. These mines generated 

significant volumes of waste rock and mine tailings (a term for finely-crushed mine 

waste rock). The type of historical mining practised in the Coromandel Peninsula can 

cause trace element enrichment of waterways through two mechanisms: direct entry of 

mine tailings to surface water, and acid mine drainage: 

• In the past, large volumes of mine tailings were disposed of directly to the 

Ohinemuri River, which enters the Waihou River, which enters the Firth of Thames. 

Specifically, the Waihou River is known to have received between 500-800 tonnes 

per annum of tailings from the Ohinemuri River for over 50 years. 

• In acid mine drainage (or AMD), minerals are released through the oxidation of the 

sulphide minerals, liberating trace elements, which are then flushed out of the mine 

workings or tailings dam by rain or groundwater into the local environment. AMD 

leachate is normally characterised by low pH, high concentrations of sulphate, 

ferrous iron (Fe 2+ aq) and trace elements (e.g. lead, copper, nickel, zinc, cadmium, 

mercury, arsenic, antimony, thallium). 

Table 1-2 lists the various streams within the Coromandel Peninsula where historical 

mining has been undertaken within the stream catchments. 

Historical mining has had some severe localised environmental impacts, most notably 

the Tui Mine tailings dam (Tay, 1981). Acid mine drainage and the subsequent release 

of metals from base metal sulphides into the aquatic environment is a significant longterm 

legacy from historical mining at this site (Rumsby, 1996). 

Mines situated in ore bodies with base metal sulphide mineralisation (such as Buffalo, 

Monowai and Cromstock within the Waiomu Catchment) could potentially have an 

impact on the water and sediment quality of the local receiving environment and 

ultimately the Firth of Thames. Many of the mines with vein deposits containing 

significant base-metal sulphides (>3%) which are listed in Table 1-2 (e.g. Komata, 

Jubilee, Tui and Waiorongomai) are located on streams which form part of the Waihou 

River Catchment. 

There is a high correlation of arsenic with iron (R 2 =0.77 with n=34; p=0.001) in several 

former mines in the Coromandel area (Hollinger, 2002). Electron microprobe data 

suggest the two main arsenic containing sulphide minerals are pyrite/marcasite which 

generally have an arsenic concentration of 0.1 to 1.6% and tetrahedrite (copper 

antimony sulphide, general formula Cu12Sb4S13) which can have an arsenic 

concentration of up to 20% (Hollinger, 2002). Pyrite (iron sulphide, FeS2) is listed as 

being ubiquitous in the Hauraki Goldfield and, therefore, pyrite would be the most 

important source of arsenic as it occurs in both the host rock and ore bearing rock 

(whereas tetrahedrite is only in the ore-bearing rock). Therefore, historic mining may be 

a source of several trace metals but especially arsenic. 

Doc # 1120743 Page 3

Table 1-2 Alteration, mineralisation and old mine workings in the Coromandel area 

(modified from Livingston 1987, including data from Jenkins 1991 and Moore et al. 

1996). 

Stream Country Rock 

Alteration 

Waitawheta Mainly unaltered or 

weakly propylitised 

Waipupu Mainly unaltered or 

weakly propylitised 

Waitekauri Mainly Strongly 

clay-altered and 

silicified 

Komata Both propylitised 

and silicified rocks 

Mangakara Mainly strongly 

clay-altered and 

silicified 

Buffalo Mainly propylitised 

or clay-altered 

Mangatoetoe Mainly unaltered; 

strong local clay 

alteration 

Waiorongomai Mainly propylitised, 

minor silication 

Jubilee Mainly clayaltered, 

minor 

propylitisation 

Mineralisation Mine Workings Mullock Tips, 

Tailings 

Some veins with 

minor base 

materials 

None known None known 

None known None known None known 

Several large 

veins, with trace 

base metals 

Several large 

veins, minor base 

metals 

Numerous small, 

commonly pyritic 

veins 

Minor veins with 

minor base metals 

Extensive (Golden 

Cross) 

Extensive 

(Komata, Te-aomarama) 

Extensive (Scotia, 

Maoriland) 

None known Extensive on 

Martha Hill 

Several large 

base-metal bearing 


Large, locally 


vein 

Cromstock Propylitised Several veins with 


Waiomu Propylitised Several veins with 


Paroquet Mainly clay altered Base-metal 

bearing vein 

Waitatia Propylitised, clayaltered 

and 

silicified 

Tui Propylitised, clayaltered 

and minor 

silicified 

Many small veins, 

several larger 


Several large 



1.1.3 Agricultural and horticultural inputs 

Extensive mine 

Workings 


workings 

Some minor 


Some mine 


(Cromstock) 



(Monowai) 



(Waitatia) 


workings (Tui) 

Present 

Extensive, contain 


Present 

Present 

Present 

Present, probably 

contain minor 

base metals 


significant base 

metals 

Not known 

Present 

Present contain 

base metals 

Present 


significant base 

metals 

Some agricultural activities involve the application to land of some materials that 

contain significant concentrations of one or more trace elements. There can be some 

loss of these elements from farms to the wider environment, including freshwater and 

marine sediments. In pastoral areas, zinc sulphate is used extensively to protect 

against the liver damage that causes facial eczema in both cattle and sheep. In 

addition, most superphosphate fertiliser contains significant amounts of several trace 

elements as impurities, most notably cadmium and fluorine (Table 1-3). Copper and 

zinc are also applied to horticultural areas in fungicide formulations. 

Page 4 Doc # 1120743

Table 1-3 Concentrations of selected elements in phosphate fertiliser, soils and rocks. 

Element Average upper 

continental 

crust 1 

Waikato soils 

(average) 2 

Phosphate rock 

(PR) (mg/kg P) 

Single 

superhosphate 

fertiliser (mg/kg 

P) 

Cadmium, Cd 0.15 0.70 1-90

• The Department of Conversation (1992) commissioned an investigation of the 

effects of Polynesian and European land use on sedimentation on Coromandel 

estuaries in 1992. As part of that work, metal concentrations were measured in a 

number of cores. 

• Research has also been conducted by Webster (1995) on chemical processes 

affecting trace metal transportation in the Waihou River and estuary. 

• More recently both the Auckland Regional Council and Environment Waikato have 

undertaken monitoring at selected sites within the Firth of Thames. One of the 

ARC sites is at Waiheke Island (Te Matuku). 1 The previous Environment Waikato 

monitoring sites around the Firth of Thames have involved shallow (0-2 cm) 

sediment samples, because the primary survey purpose was ecological monitoring, 

but have been included in the data analysis of this study (Section 3.3.1). 

• The National Water and Soil Conservation Authority (NWSCA) conducted a survey 

of stream sediment and water quality within various streams on the Coromandel 

Peninsula. The NWSCA also conducted biological monitoring within the streams 

and measured metal concentrations in shellfish and fin-fish. Unfortunately, the 

results of the NWSCA study are not usable in this study as no marine sediment 

samples were collected in that work. 

A summary of trace element concentrations reported at specific sites by Livingston 

(1987), the Department of Conversation (1992), Webster (1995) and the Auckland 

Regional Council is presented in Table 1-5. Results of previous (2003) sampling 

carried out by Environment Waikato are presented as part of this report (see Section 

3.3.1 and Appendix 6). 

In addition to these studies, Hume and Dahm (1991) of the Department of Scientific 

and Industrial Research (DSIR) used marine sediment cores to investigate the impact 

of Polynesian and European land use on sedimentation in Coromandel estuaries. 

Results revealed that trace element concentrations deposited in European times are 

elevated over pre-settlement background levels. Specific observations were as follows: 

• Concentrations of lead in pre-settlement sediments ranged from 8-18 mg/kg. In 

younger sediments lead ranged from 16-66 mg/kg, with typical values around 30 

mg/kg. Lead was typically enriched to between 1.5 to 2 times background levels, 

although lead enrichment of up 4-5 times was noted near the Waihou River Mouth. 

• Zinc concentrations were reported to range from 36-78 mg/kg in pre-settlement 

sediments to 174 mg/kg (typically 80-90 mg/kg) in younger sediments. Overall, zinc 

levels were elevated at approximately 1.5-2 times background levels (Hume and 

Dahm, 1991). 

• Background concentrations of copper ranged between 6-21 mg/kg, with post- 

European settlement concentrations ranging between 13-26 mg/kg. Copper was 

elevated only in sediments post-dating European settlement, with enrichment 

factors of between 1.5-2 noted in the core samples. 

• Arsenic was slightly elevated in post-settlement sediments to approximately 1.2-2 

times background levels. 

1 Unfortunately the ARC analytical protocol (analysis of only the sub-63 µm fraction) makes it difficult to compare 

results. The other ARC monitoring locations represent estuary environments which have been impacted by 

urbanisation. 

Page 6 Doc # 1120743

Table 1-5 Sediment quality data (mg/kg) obtained by other authors prior to this study. 

(Refer to Table 1-1 for the element name associated with each chemical symbol.) 

Location Date Ref. As Cd Cr Cu Hg Ni Pb Zn 

Waiomu Stream 

WC 1982 1 0.41

Location Date Ref. As Cd Cr Cu Hg Ni Pb Zn 

C4 (138-140 cm) 2 9.5 6.4 8.4 10.4 45.4 

Waihou River 

W11b (0-5 cm)1985 3 16.5 67.2 259 

W11b (5-10 cm) 3 25.6 100.6 200 

W11b (10-15 cm) 3 20 73 146 

W11b (15-20 cm) 3 8.2 24.6 64.3 

W13 (0-5 cm) 3 29.8 60.5 276 

W13 (5-10cm) 3 23.5 78 185 

W13 (10-15 cm) 3 17.8 75.4 179 

W13 (15-20 cm) 3 18.1 79.4 201 

W26 3 3.7 35.1 17.6 19.5 137 

Tararu Beach 

W27 1985 3 9.63 37.5 237 91.1 1478 

Te Matuku 4 

2001 3.0 1.9 38.5 

Sources: 1. Livingston (1987); 2. Department of Conservation (1992); 3. Webster (1995); 

4. Auckland Regional Council (2001). 

The authors concluded that the elevated metal concentrations are probably due to 

anthropogenic effects related to European land use. This may be due to greater 

contributions from weathering of natural minerals caused by increased surface water 

run-off following land clearance, and more direct contributions from mining operations. 

Arsenic, copper, lead and zinc are commonly associated with mine waste run-off, and 

large volumes of mine tailings that were dumped into the Ohinemuri River in early 

European times. 

Hume and Dahm’s (1991) estimates of relative enrichments of lead, zinc, copper and 

arsenic are in good agreement with those determined by comparing sediments’ quality 

in Raglan Harbour with that in the Firth of Thames, as part of this work. These 

estimates are presented in Section 3.3.1 of this report. 

Page 8 Doc # 1120743

2 Study methodology 

2.1 Sampling objectives 

As part of this work, URS was commissioned to undertake new sediment sampling of 

the eastern coast of the lower Firth of Thames for the purposes of assessing sediment 

quality and identifying the potential sources of trace elements in these sediments. 

Concentrations of eight elements were measured in all samples collected: arsenic (As), 

cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and 

zinc (Zn). A sampling programme was designed that would maximise the possibility of 

being able to discriminate between a number of possible sources of an element in 

sediments of the intertidal zone. These sources include: 

• Run-off from natural and enhanced weathering and erosion of sulphide minerals; 

• Run-off or legacy of tailings inputs from former mining sites; 

• Longer-range transport from agricultural, urban and mining areas (for example, 

large river mouths); 

• Urban sources. 

Sampling also included control sites that were not likely to have been significantly 

impacted by the above sources. Key features and catchments in the study area are 

shown in Figure 2-1. 

2.2 Sampling locations 

2.2.1 General 

Samples were collected from 11 locations on the eastern side of the Firth of Thames: 

the Waihou River mouth, the Piako River mouth, various areas around Thames, Opani 

mudflat, Tararu, Tapu, Te Puru, Thornton Bay, Waiomu, Te Mata, and Kuranui Bay 

(Table 2-1; Figure 2-2). 

Sampling by URS New Zealand Ltd was undertaken between 22 and 24 June 2005, 

with sample locations selected to characterise each of the potential sources identified. 

Targeted sampling of selected areas was undertaken, utilising a mix of composite 

sampling techniques in order to obtain representative samples and targeted depth 

sampling (Table 2-1). Sampling locations were recorded using a handheld Global 

Positioning System (GPS) unit. 

Doc # 1120743 Page 9

Figure 2-1: Context of the study area, showing catchment boundaries and other key 

features. 

Samples collected by URS NZ Ltd (Table 2-1) were collected from the Piako 

River mouth to Te Mata. In previous work also reviewed in this report, shallow 

surface (0-2 cm) samples were collected from Miranda to Te Puru. 

Page 10 Doc # 1120743

Table 2-1 Details of sediment samples collected by URS New Zealand Ltd in 2005. 

Location Sample ID 

Waihou River mouth 

Piako River mouth 

Thames pipeline 

area 

Thames mudflats 

Thames wharf & 

stormwater 

Thames deeper 

harbour 

Approximate GPS 

location (NZMS) 

Sample type 

SDB574 NZMS T12 361446 Composite made of five subsamples 








SDB810, 

811 

NZMS T12 351489 Vertical profile 

SDB580 Composite made of five subsamples 



SDB583 

SDB584 

See Figure 2-2 

Composite made of five subsamples 




SDB576 





Sampling method and 

depths 

Mix of Eckman grab sampling 

and hand sampling (plastic 

trowel). Nominal depth 10 

cm. 

Eckman grab sampling. 

Nominal depth 10 cm. 

Hand sampling (plastic 

trowel): 0-10 cm 

Push corer: 0-2 cm and 2-10 

cm 







Eckman grab sampling. 

Nominal depth 10 cm. 

Number of samples 

analysed 

2 composites 

3 composites 

3 composites, plus: 

2 individual depth 

samples. 

8 composites: 2 of 

these (SDB573 and 

SDB576) analysed 

twice for QA/QC. 

2 composites 

1 composite 

Doc # 1120743 Page 11

Table 2-1 continued... 



Location (NZMS) 

Opani mudflat SDB568 NZMS T12 334449 Composite made of five subsamples 

Tararu Stream 

Tapu Stream 

Te Puru 

Thornton Bay 



SDB598 

SDB599 

NZMS T11 341515 




SDB601 




SDB798 

SDB799 

NZMS T11 328656 




SDB801 








SDB816, 

817, 818 

and 819 

Sample type Sampling method and depths 

Vertical profile 



SDB804 

SDB615 

NZMS T12 345563 




SDB621 










Push corer: 0-2 cm, 2-10 cm, 

15-20 cm and 25-30 cm 




analysed 

1 composite 

6 composites. One 

analysed (SDB595) 

twice for QA/QC. 

6 composites 

6 composites, plus: 


samples. 

6 composites 

Page 12 Doc # 1120743

Table 2-1 continued... 



Location (NZMS) 

Sample Type Sampling method and depths 



SDB613 Composite made of five subsamples Hand sampling (plastic 



Waiomu Stream SDB615 NZMS T11 342607 Composite made of five subsamples 


SDB619A, 

619B, 619C, 

and 619D 



15-20 cm and 25-30 cm 



Te Mata 

SDB792 

SDB793 

NZMS T11 333672 






SDB795 


Kuranui Bay 

Various 








SDB812, 

813, 814 

and 815 


Composites made of five subsamples, and 

individual samples 




15-20 cm and 25-30 cm 


analysed 


analysed (SDB594) 

twice for QA/QC, 

plus: 


samples. 

6 composites 


(SDB591) sampled 

and analysed twice 

for QA/QC, plus: 


samples. 

12 composite or 

individual samples 

analysed as part of 

QA/QC 

Doc # 1120743 Page 13

Figure 2-2 Locations of sites where sediment was sampled in 2005 and old mining 

sites. Exact coordinates of the sediment sampling sites, and details about the 

numbers of samples collected, are provided in Table 2-1. 

Page 14 Doc # 1120743

2.2.2 History of the sample locations 

There are at least four possible sources of historic trace element contamination in the 

Firth of Thames: natural erosion, tailings and run-off from mining, agricultural sources, 

and urban inputs (Section 1.1). 

Thames Harbour receives input from stormwater discharged from Thames and the 

Piako, Waihou and Kauaeranga Rivers. The Waihou River is the largest river within the 

study area, and all four sources of contamination may have contributed to the 

contaminant load carried by this watercourse. Water and sediment sampling conducted 

in the catchment has previously indicated significant trace element contamination 

(Livingston, 1987). Mining in the area has involved significant workings at the following 

locations: 

• Karangahake Gorge – sections of which were mined between 1882-1933. 

• Mt Te Aroha (Tui Mine) – a base metal mine which operated between 1967 and 

1973 and is regarded as one of New Zealand’s most contaminated sites. 

• Martha Hill mine in Waihi – which is currently operational. 

Of these, inputs from the first source were substantial (500-800 tonnes of mine tailings 

per annum from the Ohinemuri River for over 50 years), impacts of Tui Mine were/are 

likely to be more localised, and present day mining operations at Martha Hill are carried 

out under resource consent conditions designed to eliminate any significant adverse 

effects. The Kauaeranga Valley was mined for cinnabar (mercuric sulphide, HgS) 

between 1899 to approximately 1906 (Watson, 1989) and the Kauaeranga River 

discharges into Thames Harbour. In addition to historic and present day mining in the 

Waihou River catchment, there are agricultural activities and urban inputs from Waihi, 

Paeroa and Te Ahora townships. 

In contrast, there is no known mining influence in the Piako River catchment area, but 

agricultural inputs are a potential source of trace elements in this area. A feature of this 

catchment is that it includes a substantial wetland, the Kapouatai peat bog. 

Historical mining activities were undertaken at several locations within the Tararu, Tapu 

and Waiomu stream catchments. The Waiomu stream catchment has received the 

most study. A number of mines operated within this valley between 1890 and 1936 

(including Paroquet, Broken Hill, Monowai and Comstock Mines). Exploration in this 

area between 1971 and 1984 revealed the presence of unmined ore bodies within this 

region (Moore et al. 1996). The ore deposit has been described as being an epithermal 

vein deposit with significant base metal sulphides (>3%). These would have the 

potential to elevate concentrations of elements in sediments if the ore body undergoes 

oxidation. In 1981, a preliminary survey of the Waiomu stream conducted by the 

Department of Scientific and Industrial Research (DSIR), National Water and Soil 

Conservation Authority (NWASCA), the Ministry of Agriculture and Fisheries (MAF) and 

the Hauraki Regional Water Board (Livingston, 1987) detected elevated concentrations 

of various trace elements (in particular, arsenic, cadmium and zinc) in the water and 

sediment of the Comstock Stream (a tributary of the Waiomu Stream). Slightly elevated 

concentrations of copper, lead and zinc have been detected in several locations within 

the Waiomu Stream water, and elevated concentrations of lead and zinc have been 

detected in Waiomu Stream sediments (Livingston, 1987). 

Kuranui Bay, the northern boundary of Thames township has been sampled in the past 

by Environment Waikato and elevated mercury and zinc concentrations were detected. 

Three control sites without any significant past mining or agricultural activities within 

their catchments were also sampled. They were Te Puru, Thornton Bay and Te Mata. 

More specific details of each location from which sediment samples were collected in 

2005 as part of this study are provided in Appendix 1. 

Doc # 1120743 Page 15

2.3 Sample types 

2.3.1 Manual grab samples 

Usually, six composite samples were tested from each of the locations sampled in 

2005 (Table 2-1). Each composite sample was made up of five sub-samples. 

In several cases, samples were collected in the intertidal area in the vicinity of a stream 

mouth. Sampling around small stream mouths was undertaken by dividing the area 

around the stream mouth into six sections (refer to Figure 2-3). 

Figure 2-3 Diagram indicating approach to collection of composite samples obtained 

from stream mouths. 

All of these samples collected for use in sediment composites were collected from the 

top 10 cm of sediments. Most epifaunal and infaunal organisms are found in the top 10 

cm of sediment. Some epibenthic species (e.g. shrimps, certain amphipods) might be 

exposed only to surficial sediments (0-1 cm) while others (e.g. bivalves and 

polychaetes) can be exposed to sediments that are several centimetres deep. 

A similar grid-type method was used at other sites (for example large river mouths and 

control sites with no stream outflows), as depicted in Figure 2-4. 

Samples were collected using a plastic hand trowel (trowel length 10 cm) and placed 

within individual plastic sample containers. Other data recorded at each site included 

site and sample identifier, site location (recorded by GPS wherever possible), and time 

and date of sample collection. 

The six composite samples for each location were prepared from the individual subsamples 

by the analytical laboratory (Hill Laboratories, Hamilton). Equipment was 

rinsed with site water between each sub-sample. 

2.3.2 Vertical profiles 

At four sites, vertical profile samples were also collected using push-corers (see Table 

2-1). Samples from three of these sites (Te Puru, Waiomu and Kuranui Bay) were 

divided into four sub-samples for analysis, to the following sediment depths: 0-2 cm, 2- 

10 cm, 15-20 cm and 25-30 cm. Samples from the fourth site (Thames stormwater 

pipeline area) were divided into two depths for analysis: 0-2 cm and 2-10 cm. 

Page 16 Doc # 1120743

Figure 2-4 Diagram indicating approach to collection of composite samples obtained 

from intertidal zone with no significant inputs. 

2.3.3 Grab sampling 

Grab samples were collected from areas covered by water by use of an Eckman grab 

sampler. Where water flow was too fast for the Eckman grab sampler to operate, at 

some locations near the Waihou River mouth, subsamples were hand-collected by 

URS staff. To ensure that grab samples were consistent and suitable for benthic 

sampling, the following criteria were utilised before the sample was accepted: 

• Sediment had not extruded from the sampler. 

• Water was still present in the sampler (i.e. the grab remained closed during 

retrieval). 

• The sediment surface was relatively flat. 

• Appropriate sediment penetration had occurred (>10 cm in silty sediments). 

If a collected sample failed to meet any of these conditions, the sample was discarded 

and another sample collected at the site. The location of consecutive attempts was 

made as close to the original attempt as possible and located in the upstream direction 

of any existing current. The rejected sample was discarded in a manner that would not 

have affected subsequent samples. 

2.4 Analysis and analytical quality assurance 

Seventy-seven sediment samples were analysed by Hill Laboratories, Hamilton. Chain 

of custody forms accompanied all samples submitted to the analytical laboratory. All 

analysis was undertaken on the sub-2 mm fraction of sediments. Total recoverable 

arsenic, cadmium, chromium, copper, mercury, nickel, lead, and zinc were analysed on 

all samples, with several samples being analysed in duplicate (Table 2-1). 

Grain size, aluminium, iron and lithium were measured in selected samples. 

Five quality assurance/quality control (QA/QC) samples were collected adjacent to 

grab samples to evaluate small-scale variations (nugget effects) and variability of both 

field and laboratory operations. 

In addition, five composited samples were re-composited by the laboratory to evaluate 

the variability in compositing. A laboratory QA/QC check was also provided. 

Doc # 1120743 Page 17

2.5 Use of ANZECC sediment quality guidelines 

To establish the degree of risk to sediment-dwelling organisms, the results from this 

survey can be compared with Australian and New Zealand Environmental 

Conservation Council (ANZECC) interim guideline values for sediment quality (ISQGs) 

for the protection of aquatic ecosystems. For each trace element, there are two 

ANZECC (2000) guidelines for sediment quality. 

• The lowest is the Interim Sediment Quality Guideline-Low (ISQG-Low) which 

represents a concentration below which adverse effects are unlikely. 

Concentrations of contaminants below the ISQG-Low pose a low level of risk to 

aquatic organisms. 

• The higher is the ISQG-High, which is a median level at which adverse effects are 

expected in half of the exposed organisms. Contaminant concentrations above the 

ISQG-High are interpreted as being reasonably likely to cause significant adverse 

effects on aquatic organisms. 

Concentrations between the ISQG-Low and ISQG-High are thought to pose a 

moderate level of risk to aquatic organisms. Concentrations of trace elements or other 

chemicals either below or above the ANZECC (2000) trigger values should not be 

thought of as safe or unsafe, but rather posing a lower or higher level of risk. This is 

because site-specific factors such as the chemical form of compound or element, 

natural background concentration, the concentration of organic matter or reduced 

sulphide compounds can all modify the toxicity of a particular compound. 

Values below the ISQG-Low do not guarantee that the concentrations are safe either 

because complex chemical mixtures of certain compounds are more toxic than their 

individual chemical components and the ANZECC (2000) guidelines are not designed 

to protect against those mixtures. Also certain compounds such as mercury have 

specific chemical forms (methylmercury, ethylmercury) which bioaccumulate in 

organisms and biomagnify up the food-chain. As bioaccumulation potential is sitespecific, 

more detailed studies are required to assess such risks. Therefore, the 

guidelines are designed to be trigger values to indicate which sites may warrant closer 

investigation. 

It should also be noted that the ANZECC (2000) guidelines are designed to protect 

aquatic ecosystem rather than to protect human health. Although ISQG-Low values are 

lower than equivalent soil quality guidelines designed to protect human health, the aim 

of this study and the guidelines used in this study is related to the protection of aquatic 

ecosystems. Therefore, no conclusion should be made on the potential human health 

risk. 

Page 18 Doc # 1120743

3 Results and discussion 

3.1 Raw results and summary statistics 

A complete list of the results for eight trace elements in all sediment samples is 

provided in Appendix 2. A summary of results obtained at each sampling site is 

presented in Table 3-1. A discussion of results from quality assurance and quality 

control samples is provided in Appendix 3. Normalised data is provided in Appendix 4, 

and a statistical summary of results for each sampled area in Appendix 5. 

Table 3-1 Summary of trace element concentrations at sampling sites and comparison 

to ANZECC (2000) sediment quality guidelines (all values in mg/kg dry weight). 

Site locations are listed from south (Waihou and Piako River mouths) to north (Te 

Mata). Bold entries exceed ANZECC (2000) ISQG-Low values; the bold italic 

entry exceeds an ISQG-High value. Refer to Table 1-1 for the element name 

associated with each chemical symbol. 

As Cd Cr Cu Hg Ni Pb Zn 

ANZECC (2000) ISQG-Low 20 1.5 80 65 0.15 21 50 200 

Location 

Waihou River mouth (N=2) 

Piako River mouth (N=3) 

Opani mudflat (N=1) 

Thames wharf & stormwater (N=2) 

Thames mudflats & harbour (N=9) 

Thames pipeline (N=5) 

Kuranui Bay (N=9) 

Tararu Stream (N=7) 

Thornton Bay (N=6) 

Te Puru (N=8) 

Waiomu Bay (N=8) 

Tapu (N=6) 

Te Mata (N=6) 

Minimum 

Maximum 

Average 

Standard deviation 

95% confidence error 

ISQG-High 70 10 370 270 1.0 52 220 410 

7.3 0.11 30.8 9.9 0.22 9.2 26.5 91.7 

9.2 0.23 24.3 10.4 0.32 8.5 29.5 104 

7.5 0.17 25.1 9.7 0.27 8.4 28.9 98.5 

14.6 0.11 24.8 12.0 0.28 8.2 27.8 90 

14.2 0.09 24.8 13.8 0.35 8.5 20.5 73.8 

106 0.40 24 16.9 0.47 8.7 35.6 149 

36.1 0.41 22.7 16.4 0.76 8.1 29.9 146 

24.5 0.17 16.3 23.0 0.18 6.0 25.6 99.1 

23.0 0.03 21.5 17.8 0.08 8.1 22.2 72.7 

24.3 0.05 21.7 21.5 0.09 7.8 20.4 72.2 

27.3 0.10 26 31.8 0.09 8.8 35.3 82.3 

21.2 0.02 19.6 14.0 0.12 8.3 8.15 54.4 

26.7 0.03 20.7 11.3 0.15 8.6 29.9 63.4 

7.3 0.02 16.3 9.7 0.08 6.0 8.15 54.4 

106 0.41 30.8 31.8 0.76 9.2 35.6 149 

26.3 0.15 23.2 16.0 0.26 8.3 26.2 92.0 

25.5 0.13 3.5 6.4 0.19 0.8 7.2 28.6 

16.0 0.08 2.2 4.0 0.12 0.5 4.6 18.0 

Doc # 1120743 Page 19

3.2 Comparison with sediment quality guidelines 

3.2.1 Comparison to the ISQG-Low 

To ascertain the potential ecological significance of the data collected from all the sites, 

the average concentrations for each trace element at each site were divided by 

sediment quality guidelines, the ANZECC (2000) ISQG-Low and ISQG-High (for more 

about which refer to Section 2.5). Results of the comparison of average concentrations 

with the ISQG-Low are provided in Table 3-2. 

Table 3-2 Summary results expressed as a fraction of the lowest ANZECC (2000) 

sediment quality guideline (the ISQG-Low). Refer to Table 1-1 for the 

element name associated with each chemical symbol. 


Waihou River mouth 0.37 0.07 0.39 0.15 1.47 0.44 0.53 0.46 

Piako River mouth 0.46 0.15 0.30 0.16 2.13 0.40 0.59 0.52 

Opani mudflat 0.38 0.11 0.31 0.15 1.80 0.40 0.58 0.49 


stormwater 0.73 0.07 0.31 0.18 1.87 0.39 0.56 0.45 

Thames mudflats & 

harbour 0.71 0.06 0.31 0.21 2.33 0.40 0.41 0.37 

Thames pipeline 5.30 0.27 0.30 0.26 3.13 0.41 0.71 0.75 

Kuranui Bay 1.81 0.27 0.28 0.25 5.07 0.39 0.60 0.73 

Tararu Stream 1.23 0.11 0.20 0.35 1.20 0.29 0.51 0.50 

Thornton Bay 1.15 0.02 0.27 0.27 0.53 0.39 0.44 0.36 

Te Puru 1.22 0.03 0.27 0.33 0.60 0.37 0.41 0.36 

Waiomu Bay 1.37 0.07 0.33 0.49 0.60 0.42 0.71 0.41 

Tapu 1.06 0.01 0.25 0.22 0.80 0.40 0.16 0.27 

Te Mata 1.34 0.02 0.26 0.17 1.00 0.41 0.60 0.32 

Average 1.32 0.10 0.29 0.25 1.73 0.39 0.52 0.46 

Two elements exceeded the lowest sediment quality guideline value (the ANZECC 

(2000) ISQG-Low) in surface sediments at some locations (Tables 3-1 and 3-2): 

• Average arsenic concentrations in sediments exceeded the ISQG-Low at eight 

(62%) of the thirteen sites. Arsenic averaged 1.32 times the ISQG-Low (20 mg/kg) 

and 38% of the ISQG-High (70 mg/kg). 

• Average mercury concentrations in sediments also exceeded the ISQG-Low at 

eight (62%) of the thirteen sites. Mercury averaged about 1.7 times the ISQG-Low 

(0.15 mg/kg) in surface samples (range 0.53 to 5.1 times). Mercury averaged about 

26% of the ISQG-High. 

These results put arsenic and mercury in the Firth of Thames sediments at a point 

where they may be starting to have adverse effects on some sediment-dwelling 

organisms, at some locations (Section 2.5). 

Mercury exceeded guidelines at the southern sites and the four sites immediately north 

from Thames, but remained below guidelines at the five most northern sites. The 

highest concentrations were found at Kuranui Bay, and the nearby pipeline sampling 

site in northern Thames. The Kuranui Bay site is also atypical of the other sites 

sampled to depth, in that concentrations at depth were significantly greater than those 

at the surface. After this localised hotspot is excluded, the highest mercury 

concentrations are associated with the Thames mudflats and the Piako River mouth 

(Tables 3-1 and 3-2). Bioaccumulation of the mercury up the food chain is the main risk 

associated with elevated concentrations of mercury. The ANZECC (2000) guideline 

levels are not designed to protect against this type of risk, therefore compliance or noncompliance 

with the ANZECC (2000) trigger levels does not provide any information 

Page 20 Doc # 1120743

about potential risks to wildlife, or people consuming seafood. A discussion about 

potential sources of mercury appears in Section 3.6.5. 

3.2.2 Comparison to the ISQG-High 

Results of the comparison of average concentrations with the ISQG-High are provided 

in Table 3-3. 

Table 3-3 Summary results expressed as a fraction of the upper ANZECC (2000) 

sediment quality guideline (the ISQG-High). 


Waihou River mouth 0.10 0.01 0.08 0.04 0.22 0.18 0.12 0.22 

Piako River mouth 0.13 0.02 0.07 0.04 0.32 0.16 0.13 0.25 

Opani mudflat 0.11 0.02 0.07 0.04 0.27 0.16 0.13 0.24 


stormwater 0.21 0.01 0.07 0.04 0.28 0.16 0.13 0.22 

Thames mudflats & 

harbour 0.20 0.01 0.07 0.05 0.35 0.16 0.09 0.18 

Thames pipeline 1.51 0.04 0.06 0.06 0.47 0.17 0.16 0.36 

Kuranui Bay 0.52 0.04 0.06 0.06 0.76 0.16 0.14 0.36 

Tararu Stream 0.35 0.02 0.04 0.09 0.18 0.12 0.12 0.24 

Thornton Bay 0.33 0.00 0.06 0.07 0.08 0.16 0.10 0.18 

Te Puru 0.35 0.01 0.06 0.08 0.09 0.15 0.09 0.18 

Waiomu Bay 0.39 0.01 0.07 0.12 0.09 0.17 0.16 0.20 

Tapu 0.30 0.00 0.05 0.05 0.12 0.16 0.04 0.13 

Te Mata 0.38 0.00 0.06 0.04 0.15 0.17 0.14 0.15 

Average 0.38 0.01 0.06 0.06 0.26 0.16 0.12 0.22 

Only one site was found to exceed the ISQG-High, on average, and this was for only 

one element (Table 3-3). This was for arsenic in the sample collected from the vicinity 

of the Thames pipeline. 

• On closer examination it is evident that the high result for arsenic in this sample 

(125 mg/kg) is the result of a single result containing 433 mg/kg arsenic in one of 

the sub-samples (Appendix 2). This result is considered to be a sampling outlier 

because the sample was collected within an area of sediment containing an ironpan 

layer which is known to accumulate arsenic. Therefore, although the 433 mg/kg 

is likely to be close to the true concentration of arsenic in this sample (it is not likely 

to be an analytical outlier), the result does not adequately represent the 

composition of the sediment in this location, but the arsenic concentration of an 

iron-pan sample, making it a sampling outlier. 

When the Thames pipeline outlier is removed, no site is found to exceed the ISQG- 

High, on average, for any element (Tables 3-1 and 3-3). 

3.2.3 Key findings 

• Arsenic and mercury concentrations in the Firth of Thames coastal sediments are 

not extremely high, but are at a point where they may have adverse effects on 

some sediment-dwelling organisms at some locations. 

• There is an apparent hotspot at Kuranui Bay and an adjacent site in northern 

Thames. A further assessment of this is provided in Section 3.5.4. 

• Mercury and arsenic do not behave like one another. Excluding the 

Kuranui/northern Thames hotspot, the highest mercury concentrations are at the 

southern sites, where arsenic concentrations are lowest. By contrast, the highest 

arsenic concentrations are along the eastern coast. 

Doc # 1120743 Page 21

3.3 Comparison between the Firth of Thames and 

other areas 

3.3.1 Comparison to coastal sediments in Raglan Harbour 

Trace element concentrations in the Firth of Thames sediments can be compared with 

what would be expected in clean Waikato soils, crustal abundances, and data for 

Raglan Harbour. The most relevant comparison is between samples from a previous 

Environment Waikato study of sediments from the Firth of Thames and Raglan 

harbour. In that survey, sediment samples (0-2 cm) had been collected in October 

2003 as part of an ecological survey from five locations in Raglan Harbour (Ponganui 

Creek, Whatitirinui Island, Te Puna Point, Okete Bay and Haroto Bay), and five 

locations in the southern Firth of Thames (Kaiaua, Miranda, Thames, Kuranui Bay and 

Te Puru). 

At each of the ten locations, five 2 m x 2 m squares had been sampled, and each of 

these consisted of 12 sub-samples. Composites from each of the ten locations were 

analysed for 33 chemical elements, organic contaminants, and a range of major 

variables. Full results are archived and summarised separately. 2 Results for the 33 

trace elements, total organic carbon (TOC) and percent dry matter at the ten locations 

are provided in Appendix 6 of this report. Average concentrations of selected elements 

are presented in Table 3-4. In this table, five elements are identified as crustal 

indicators. This term is used to describe elements whose only significant source is 

likely to be the geological parent matrix of the sample. 3 

Comparison of the results for Raglan Harbour with those for the Firth of Thames and 

those for clean Waikato soils shows that: 

- Concentrations of all elements in the Raglan sediments with the possible exception 

of lithium 4 are (analytically) equivalent to or below those expected for clean Waikato 

soils. 5 

- Comparison of means and medians between the two harbours shows very similar 

concentrations of elements that could be taken as crustal markers: aluminium, iron, 

lithium, chromium, and nickel. 

These two factors suggest that Raglan sediments (0-2 cm) can be used as a control 

site to represent clean Firth of Thames sediments. Average concentrations found in 0-2 

cm sediments from around each harbour, and relative enrichment ratios for the Firth of 

Thames (0-2 cm) sediments are shown in Table 3-4. 

Relative to Raglan, the Firth’s sediments appear to be enriched in mercury, lead, 

cadmium, copper, zinc and arsenic (in this order). 6 

2 

Data set: Environment Waikato document 922343; summary of results as an Environmental Indicator: 

http://www.ew.govt.nz/enviroinfo/indicators/coasts/waterquality/co12a/report.htm 

3 

The term ‘crustal’ reflects the fact that soils and sediments are produced through weathering of the earth’s crust. 

4 

Higher concentrations of lithium might be expected in coastal sediments than terrestrial soils due to the presence of 

elevated lithium in seawater. 

5 

Average background concentrations of these elements in Waikato surface (0-10 cm) soils by pseudo-total strong 

acid extraction are currently estimated to be aluminium: 36,000 mg/kg; arsenic: 8 mg/kg; cadmium: 0.2 mg/kg; 

chromium: 15 mg/kg; copper: 25 mg/kg; iron: 24,000 mg/kg; lithium: 8 mg/kg; mercury: 0.16 mg/kg; nickel: 7 mg/kg, 

lead: 15 mg/kg, zinc: 67 mg/kg (based on up to 50 soils sampled from a range of soil types; some sites omitted for 

arsenic, copper and lead to account for contamination from horticultural sprays; cadmium estimate based on 

reserves and forest areas with limited phosphate fertiliser use). 

6 

This finding that the Firth of Thames sediments are likely to be enriched in mercury, lead, cadmium, copper, zinc 

and arsenic should not be taken to imply that concentrations of these elements in Firth of Thames sediments are at 

unacceptably high levels for ecosystem health. Comparisons to ANZECC (2000) guideline values for the elements 

in sediments are provided in Section 3.2. 

Page 22 Doc # 1120743

Table 3-4 Average concentrations of eleven elements in shallow surface (0-2 cm) 

sediments collected from Raglan Harbour and the Firth of Thames in earlier 

work, and comparison of averages between the two areas. 

Element 

Average concentration in 0-2 

cm sediment samples 

Ratio of means 

(Firth of Thames / 

Raglan) 

Interpretation for Firth 

of Thames 

Firth of Thames Raglan 

Aluminium 9300 10300 0.9 crustal indicator 

Iron 24700 23100 1.1 crustal indicator 

Chromium 14.9 14.4 1.0 crustal indicator 

Nickel 6.23 8.06 0.8 crustal indicator 

Lithium 14.3 13.5 1.1 crustal indicator 

Arsenic 12.7 8.34 1.5 potential contaminant 

Cadmium 0.087 0.024 3.6 potential contaminant 

Copper 12.5 6.76 1.9 potential contaminant 

Mercury 0.289 0.026 11.1 potential contaminant 

Lead 24.3 6.09 4.0 potential contaminant 

Zinc 80.5 47.7 1.7 potential contaminant 

Results from the earlier shallow (0-2 cm) sampling are in general keeping with the 

results from this survey (surface grab or 0-10 cm sampling), which indicate varying 

degrees of enrichment in Firth of Thames sediments for the same six elements. As 

noted earlier (Section 3.2), in this work mercury and arsenic are the two elements most 

likely to exceed sediment quality guidelines at some sites. Based on 0-2 cm sediments, 

average Firth sediments contain about 4.4 mg/kg more arsenic and 0.26 mg/kg more 

mercury than the Raglan Harbour sediments that were sampled to the same depth 

(Table 3-4). 

In Section 3.5.4, evidence is presented that the Kuranui Bay site in the Firth of Thames 

is an area of specific localised contamination by some elements, above that occurring 

at other sites. A case might be made that this site should be excluded from the 

comparison between the Firth of Thames and Raglan sediments, on the basis that it is 

a more localised hotspot. Removal of this site from the 0-2 cm sediment data set has 

no effect on which elements are identified as crustal indicators, but does have an 

impact on the relative enrichments of mercury, cadmium and zinc in Firth of Thames 

sediments relative to those of Raglan. Mercury drops from an average enrichment 

factor of 11 to 7; cadmium drops from 4 to 1.5, and zinc drops from 1.7 to 1.2. 

Enrichment ratios for the other elements are not substantially altered. 

In absolute mass terms, the most enriched element is zinc. Including the Kuranui Bay 

sample, the shallow Firth of Thames samples also contain (on average) 33 mg/kg more 

zinc than Raglan sediments. Excluding it, the Firth sediments still contain 9.2 mg/kg 

more zinc than those of Raglan Harbour. 

The relative enrichment results determined in this work are in good agreement with 

previous estimates of the effect of Polynesian and European land use on lead, zinc, 

copper and arsenic concentrations in Firth of Thames sediments, which are reviewed in 

Section 1.2. For example, based on sediment cores the DSIR estimated that arsenic 

was elevated 1.2–2 times in more recent sediments compared to old sediments in the 

Firth of Thames samples (Hume and Dahm, 1991). This compares well with an 

average enrichment of 1.5 times compared with the uncontaminated sediments of 

Raglan Harbour (Table 3-4). 

Overall, comparison between harbours provides evidence that arsenic, cadmium, 

copper, mercury, lead and zinc are enriched in coastal sediments of the Firth of 

Doc # 1120743 Page 23

Thames. This result is in agreement with earlier work carried out on four of these 

elements by the DSIR (Section 1.2). The results for mercury and cadmium are new. 

• Relative to its expected background concentration, the most highly enriched of the 

eight elements measured in this study is mercury. On average, mercury appears to 

be present in Firth of Thames shallow (0-2 cm) surface sediments at approximately 

7-11 times its usual concentration, with the difference depending on whether or not 

the hotspot associated with Kuranui Bay is excluded. 

• In absolute mass terms, the most highly enriched element is zinc. Firth of Thames 

sediments contain between 9-33 mg/kg more zinc than would otherwise be 

expected, with the difference depending on whether or not the sample from Kuranui 

Bay is excluded. 

3.3.2 Comparison of this survey’s data with historical data 

In samples collected in the URS survey (Table 3-1), lead concentrations ranged from 

8.1 mg/kg (Tapu) to 65.3 mg/kg (Waiomu Bay), with typical values being around 25 

mg/kg, and an overall average of 26 mg/kg. These values are higher than pre- 

European concentrations reported in the DSIR study, again indicating some lead 

enrichment. The low concentrations of lead found in Tapu were in a similar 

concentration range as those found in pre-settlement times. Tapu had the lowest 

sediment concentrations of all metals except for arsenic and mercury (Table 3-1 and 

Appendix 2). 

Arsenic concentrations ranged from 8.7 mg/kg around the Thames wharf to 433 mg/kg 

near the stormwater pipe outfall, with the latter result assumed to be a nonrepresentative 

sampling outlier (Section 3.2.2). The typical arsenic concentration was 

around 23 mg/kg (overall average 26 mg/kg), which is elevated above natural 

concentrations (typically 6-8 mg/kg). Higher concentrations still were found at Kuranui 

Bay and near the Thames pipeline. 

Cadmium concentrations were elevated around Kuranui Bay and the Thames pipeline, 

but they also appeared slightly enriched around the Piako River Mouth. 

Copper concentrations ranged between 7.6 mg/kg (Waihou River Mouth) to 41.6 mg/kg 

Waiomu Bay, with typical levels of around 16 mg/kg being found at most sites. For 

copper, many sites fell within the background concentration range reported in the DSIR 

study. However both Waiomu Bay and Tararu Stream showed signs of enrichment of 

copper compared with pre-settlement samples and other samples collected during this 

study. 

Mercury concentrations varied over the study area from 0.05 mg/kg at Te Puru up to 

1.58 mg/kg within Thames Harbour. Typically, mercury concentrations tended to be 

approximately 0.1 mg/kg at the northern sites, but elevated concentrations were 

observed around the Thames Harbour, Waihou and Piako River mouths, Thames 

sewer outfall and Kuranui Bay. Mercury concentrations previously reported for 

sediments from Miranda and Kaiaua were approximately 0.05 mg/kg, which probably 

represents the pre-European background concentration. 

Zinc concentrations ranged from 51.5 mg/kg (at Tapu) to 206 mg/kg (Kuranui Bay), 

although typically the concentration of zinc within the Firth of Thames is between 70-80 

mg/kg. These concentrations are similar to those found in the DSIR report, where the 

additional zinc was attributed to widespread anthropogenic enrichment (Hume and 

Dahm, 1991). The zinc concentration at Miranda and Kaiaua was between 37-43 

mg/kg, which indicate about 1.5-2 times enrichment of zinc in sediments at most sites. 

Nickel and chromium concentrations were very similar between sites. These elements 

are not significant contaminants in acid mine drainage discharges from the Tui Mine 

Page 24 Doc # 1120743

Tailings (Rumsby, 1996) and are unlikely to be a significant component of discharges 

from historic mining operations in this section of the Coromandel Peninsula. 


Arsenic, cadmium, copper, mercury, lead and zinc are enriched in coastal sediments of 

the Firth of Thames, relative to concentrations present before Polynesian and 

European colonisation, and relative to reference concentrations in sediments from 

Raglan Harbour. 

Relative to its expected background concentration, the most highly enriched element is 

mercury, which may be present at approximately seven times its usual concentration 

(excluding the anomaly at Kuranui Bay). In absolute mass terms, the most highly 

enriched element is zinc, with Firth of Thames sediments containing about 10 mg/kg 

more zinc than would otherwise be expected. 

3.4 Small scale variability 

3.4.1 Assessment 

Five samples were collected within a one metre radius of each other on the Thames 

Harbour mudflats, to evaluate small-scale variations present in the sediments. Results 

are provided in Table 3-5. 

Table 3-5 Small scale variability in chemical composition (mg/kg). (%RSD stands for 

per cent relative standard deviation, also known as the Coefficient of Variation). 

Sample No As Cd Cr Cu Hg Ni Pb Zn 

SDB576A 14.7 0.05 28 16.6 0.2 9 16 70.2 

SDB576B 16.3 0.04 21.7 14.6 0.17 7.2 12.5 52.5 

SDB576C 17.8 0.04 30.7 17 1.58 8.8 15.3 64.3 

SDB576D 15.8 0.04 24.4 16.1 0.2 8.5 13.2 55.8 

SDB576E 12.7 0.07 30.3 16 0.22 9.4 17.5 72.9 

Average 15.5 0.05 27.0 16.1 0.5 8.6 14.9 63.1 

Standard deviation 1.9 0.01 3.9 0.9 0.6 0.8 2.0 8.9 

Mean 15.5 0.05 27.0 16.1 0.5 8.6 14.9 63.1 

95% confidence error 2.4 0.02 4.8 1.1 0.77 1.0 2.5 11.0 

%RSD 12.3 20.8 14.4 5.6 127 9.3 13.4 14.1 

These results show reasonable variation in the per cent relative standard deviations 

(%RSDs) for both mercury (127%) and cadmium (20%). 

• In the case of cadmium, this is due to the fact that the measured concentrations are 

reasonably close to instrumental detection limits (0.01 mg/kg). Under these 

conditions the signal-to-noise ratio is naturally higher and results in a high %RSD 

as a result of measurement uncertainty. 

• In the case of mercury, some variability may also be due to this cause, but in 

addition, sample SDB576C has an abnormally high concentration relative to the 

other samples. This is probably due to a grain of cinnabar (HgS) or another mineral 

being present in the sub-sample that was tested (typically less than 0.5 grams of 

the total), a well-known phenomenon known as a ‘nugget effect.’ If sample 

SDB576C was removed from the dataset the %RSD for mercury would be 10.4%. 

Doc # 1120743 Page 25

The variability in the chemical composition observed for the other metals was between 

5.6% for copper to 14.4% for chromium, which would be considered normal. 

Overall, the results indicate that samples are reasonably likely to be representative of 

each sampling location, but finding an occasional high value caused by presence of a 

mineral grain (a nugget effect) would not be unusual. The results give tentative 

evidence for presence of grains of mercury-rich minerals in sediments of some areas of 

the Firth of Thames. 

3.4.2 Key finding 

Samples appear to be representative of sampled areas. There is some tentative 

evidence for the presence of grains of mercury-rich minerals (e.g. cinnabar, HgS) in 

sediments of some areas of in the Firth of Thames. 

3.5 Comparisons between sites in the Firth of 

Thames 

3.5.1 Statistical analysis and data normalisation 

Data collected at each site underwent statistical analysis to determine means, 

medians, standard deviations, confidence intervals and normality. Student’s t-tests 

were used to compare sites with each other. A range of comparisons were made both 

with and without grain size normalisation, and elemental normalisation. 

Grain size-normalisation requires separation of the sediment into two or more size 

fractions prior to analysis (de Groot et al., 1982; Salomons and Förstner, 1980). 

Separation techniques have long been used in geochemical exploration to enhance the 

signal from ore deposits and are becoming more widespread in environmental 

investigations. These techniques generally involve the separation of fractions less than 

62.5 μm by sieving and settling methods. Ackermann et al. (1980) and Louma (1990) 

reviewed several methods for separation of fine sediment and recommended a 62.5 

μm cutoff size. Many studies have been carried out using this sediment fraction, and 

data are comparable if the extraction techniques and analytical methods are similar 

(e.g. Birch and Taylor 1999). Separation at 62.5 μm and 2 mm is common (Klamer, 

1990) and represents the grain size and hydrodynamic divisions of mud, sand and 

gravel, respectively, but other mesh sizes are also used. NOAA (1988) simply divided 

total sediment concentrations by the proportion of the fine (20% mud. In this study, post-analysis correction was 

conducted on the

3.5.2 Local historical mining sites versus control sites 

Three sites selected on the basis of local mining in their catchments were Tararu, 

Waiomu, and Tapu. The three control sites which are not known to have had local 

mining in their catchments were Thornton Bay, Te Puru and Te Mata (Section 2.2.2). 

Data from the three control sites was consistent enough to be pooled in a single control 

data set for use in student’s t-tests. Results for Tararu, Waiomu, and Tapu were 

compared with this pooled data from the control sites. Results are shown in Table 3-6. 

Table 3-6 Results of Student’s t-tests between Tararu, Waiomu, and Tapu and pooled 

data from the three mining control sites. Mean values are in mg/kg (dry 

weight), and have not been normalised to any other variable. Ratios of means are 

unitless. 

Mean of control sites (Te 

Puru, Thornton and Te Mata) 

(N=20) 


24.7 0.040 21.3 17.3 0.105 8.16 23.8 69.7 

Mean at Tapu (N=6) 

Pooled t-test: concentration 

21.2 0.022 19.55 13.98 0.123 8.333 8.147 54.38 

significantly higher than 

control site 

No No No No No No No No 

Probability value (p) 0.9958 0.8942 0.9444 0.9525 0.1727 0.2937 1.0000 1.0000 

Ratio of means where 

significant 

- - - - - - - - 

Mean at Waiomu (N=8) 


27.3 0.089 26.0 31.8 0.079 8.80 35.3 82.3 


control site 

Yes Yes Yes Yes No Yes Yes Yes 



significant 

1.1 2.2 1.2 1.8 - 1.1 1.5 1.2 

Mean at Tararu (N=7) 


24.5 0.166 16.3 23.0 0.179 6.04 25.6 99.1 


control site 

No Yes No Yes Yes No No Yes 



significant 

4.2 - 1.3 1.7 - - 1.4 

Interpretive note 

It should be borne in mind that this comparison relates to effects that may have been 

caused by small mine operations in a specific sub-catchment, which are substantive 

enough to stand out above more general trace element enrichments in Firth of Thames 

sediments. At the more general level, the control sites themselves show evidence of 

trace element enrichment. For example, there is consistent arsenic enrichment at all 

three of the control sites, which have an average arsenic concentration in surface 

sediments of 24.7 mg/kg (dry weight). 

Tapu 

Tapu is the northernmost of the three sites selected on the basis of historical mining 

activity. No statistically significant difference was found in trace element concentrations 

between Tapu and the control sites. Results for Tapu were indistinguishable from those 

of the control sites, including the presence of generally elevated arsenic (Table 3-6). In 

the case of Tapu, there is no specific evidence for a localised effect from historic 

mining in the Tapu catchment which is over and above other enrichments in the Firth of 

Thames sediments. 

Doc # 1120743 Page 27

Waiomu Bay 

Moving down the coast, Waiomu Bay was the middle of the three mining areas 

selected. Anecdotal evidence has indicated that iron staining occurs periodically on the 

foreshore which indicates that metals may still be discharged from the old Monowai 

mine and precipitate on the foreshore. By contrast with Tapu, and by comparison with 

the control sites, sediments of Waiomu Bay are significantly enriched with seven of the 

eight trace elements tested – all but mercury. 

However, although statistically significant, in most cases these enrichments are not 

substantial. Only one of the elements exceeds its expected concentration by a factor of 

more than two, and this is cadmium (Table 3-6). Adding to this picture, contaminant 

depth profiles taken at Waiomu Bay show no significant difference in trace element 

concentrations with depth down to 30 cm (Appendix 2). In this regard, sediments of 

Waiomu Bay are no different from those of the control site of Te Puru (Appendix 2). 

This lack of a change with depth, coupled with the fact that the local enrichments 

(relative to control sites) are only marginal or modest, suggests that the results from 

Waiomu Bay may reflect slight differences in sediment composition this area, rather 

than being caused by run-off from mining in the catchment. 

Tararu 

The southernmost of the three areas was Tararu. Relative to controls, this site shows 

statistically significant enrichment in half of the elements: cadmium, mercury, zinc and 

copper. However, once again, the magnitude of enrichment is modest with the 

exception of cadmium. The apparent enrichment of mercury is also caused by one 

moderately elevated result, which could be treated as a data outlier (Appendix 2). 

Depth samples were not collected at Tararu. The results for this area do not provide 

any convincing evidence for a more than minor localised effect from past mining. 

Benefits of normalisation 

It was found that normalisation of data for any of the sites did not assist in 

interpretations of potential sources, but tended only to confuse matters. In the case of 

Waiomu Bay, normalising the data against lithium suggested that arsenic, copper, lead 

and zinc may be moderately enriched. Normalising using iron concentrations 

suggested that only arsenic, copper and lead were enriched. Normalising using 

aluminium suggested that most of the elements were enriched at this site. Elemental 

normalisation did not indicate that any of the other sites have undergone any 

geochemical enrichment. 

Effect of localised mining sites - overall picture 

Overall, the results suggest that either local mining operations may not have 

contributed substantially to trace elements in the nearest Firth of Thames sediments, or 

that any local impact was lost against more significant contributions from other sources, 

which also impacted the mining control sites. 

Data gathered in this work, and the former DSIR study, both indicate that trace element 

concentrations in coastal sediments of the lower Firth of Thames sediments are 

generally elevated above their natural levels (Section 3.3). Of the sources discussed in 

Section 1.1, those most likely to have caused this general elevation are enhanced 

weathering and erosion following land clearance, and the impact of historic mining 

operations which involved disposal of large volumes of tailings directly to the Ohinemuri 

River. An explanation of the lack of any evidence of metal enrichment from historical 

mining (except perhaps at Waiomu Bay) may be due to the relatively low volumes of 

metals being discharged from the mining point sources, relative to contributions from 

these other sources. This would lead to the mining discharges being diluted by much 

Page 28 Doc # 1120743

larger volumes of sediment from elsewhere within the catchments which are also 

elevated in arsenic and copper. 

The results of this work, with little evidence for significant localised impacts from former 

mining sites in the Coromandel Peninsula itself, are in agreement with the work of 

Craw and Chappell (2000). These authors examined metal redistribution in historic 

Coromandel mine wastes and found that the decomposition of sulphides in such 

wastes is slow, resulting only in millimetre-scale alteration zones over one hundred 

years. The authors concluded that the combination of slow decomposition, localised 

incorporation of metals into iron oxyhydroxide cements, low permeability, and almost 

constant water saturation in a moist climate ensures that metal discharges into the 

environment from wastes in the Coromandel area are generally at low levels. 

Cadmium and zinc 

The only results which stand out in this comparison are apparent trends for both 

cadmium and zinc. Relative enrichment factors for these two elements (compared with 

the mining control sites) increase as the sampling location moves further south. The 

three control sites cover a reasonable geographical spread of the area north of 

Thames, and their pooled results can be taken to represent averages of this area. 

Cadmium and zinc appear to become more enriched the closer the samples are 

collected from the Waihou/Piako river mouths (Table 3-7 and Figure 3-1). In this 

comparison, data for Kuranui Bay and Thames is excluded because these sites show 

strong evidence of localised trace element enrichments, which may be associated with 

fill and reclamation (Section 3.5.4). 7 

Table 3-7 Apparent enrichment of cadmium and zinc in sediments relative to the 

three mining control sites moving from south to north. Numbers are 

enrichment ratios, representing the average concentration in sediments at a 

given site divided by the pooled average for the mining control sites. 

Location 

Enrichment 

ratios 

Cd Zn 

Waihou and Piako river mouths 

and Opani mudflat 

4.5 1.42 

Tararu 4.2 1.4 

Waiomu 2.2 1.2 

Tapu 0.5 0.8 

Enrichment ratio for cadmium 

5 

4 

3 

2 

1 

0 

Waihou/Piako Tararu Waiomu Tapu 

Location (south to north) 

Figure 3-1 Apparent enrichment of cadmium relative to the three mining control sites 

moving from south to north. 

7 The enrichment factors for cadmium at Kuranui Bay and the Thames stormwater pipeline are 10.3 and 10.0 

respectively, whereas the Thames mudflats, deep harbour, wharf and near-shore stormwater sites (combined) show 

an enrichment factor of 2.2. 

Doc # 1120743 Page 29

This trend is tentative evidence of long-range transport of these two elements 

dominating over local sources. Spatial trends in element concentrations in the Firth of 

Thames sediments are examined in further detail in Section 3.6. 

3.5.3 Seaward versus landward 

Trace element concentrations in samples collected from nearest the Firth of Thames 

water (seaward) were compared with those from samples collected closer to the land 

(landward) (see Figures 2-3 and 2-4 for an indication of each class of sample). 

Data was compared on a site-by-site basis, and by grouping data into all seaward sites 

versus all landward sites. When grouping data, this type of comparison requires a 

paired t-test rather than data pooling, to ensure that real differences are not lost in the 

noise caused by changes in concentrations between sampling sites. There were twenty 

data pairs for each element in the paired t-test. 

On a site-by-site basis, and with or without normalisation, no statistical difference (to 

p=0.05) was found between seaward and landward samples. Using grouped data, 

marginal differences emerged for copper (p

Table 3-8 Results of Student’s t-tests between Thames and Kuranui Bay sites and 

other coastal sites. Mean values are in mg/kg (dry weight), and have not been 

normalised to any other variable. Ratios of means are unitless. 


Mean of reference sites (N=41) 24.6 0.069 21.1 20.6 0.119 7.95 24.0 74.9 

Mean at Kuranui Bay (N=9) 36.1 0.407 22.7 16.4 0.763 8.10 29.9 146 


significantly higher than reference 

sites Yes Yes No No Yes No No Yes 


Ratio of means where significant 1.5 5.9 6.4 1.9 

Mean at Thames stormwater 

pipeline (N=5, except N=4 for 

arsenic due to one outlier being 

removed) 24.4 0.396 24.0 16.9 0.470 8.74 35.6 149 



sites No Yes No No Yes No Yes Yes 


Ratio of means where significant 5.7 4.0 1.5 2.0 

Mean at Thames mudflats, deep 

harbour, wharf and stormwater sites 

(N=11) 

14.2 0.084 24.8 14.0 0.355 8.48 19.9 72.4 



sites No No Yes No Yes No No No 


Ratio of means where significant 1.2 3.0 

Trace element enrichments in the area of the Thames stormwater discharge pipeline 

and Kuranui Bay are very similar (Table 3.8), implying that a common source is 

responsible for the locally elevated mercury, cadmium, and zinc. Of these, mercury and 

cadmium are the most enriched. 

A possible natural source of mercury is the cinnabar (mercuric sulphide) deposit 

situated in the Kauaeranga Valley; Thames Wharf is located on the Kauaeranga River. 

The Kauaeranga Valley was mined for cinnabar between 1899 and approximately 1906 

(Section 2.2.2). However, based on subsequent review of historic information from this 

area, the most likely source is mine tailings and other municipal or industrial fill that has 

been deposited in the immediate vicinity as part of historic land reclamation. This is 

because the area between the two anomalous sampling points (Kuranui Bay and the 

Thames pipeline) is a large area of reclaimed land known as the Moanataiari 

reclamation. The Moanataiari subdivision of the town of Thames and the Thames 

landfill both sit on this reclaimed land, as shown in Figure 3-2. 

Doc # 1120743 Page 31

Figure 3-2 Aerial photograph of the Moanataiari reclamation dated 24 June 2006. 

Imagery sourced from Terralink International Ltd (TIL) 2006 and is the property 

of TIL and the Waikato Regional Aerial Photography Service (WRAPS) 2006. 

Copyright Reserved. 

The following summary of the reclamation appears in a report prepared for the Ministry 

for the Environment (2001) in relation to the effect of climate change on coastal areas: 

“The Moanataiari reclamation was formed progressively from the turn of the 

century, initially by dumping mine tailings and mullock 8 over intertidal flats. 

Dumping dredgings from the port further reclaimed the area, which was then 

capped with a raft of weathered rock and clay from the hills under more 

controlled conditions in the mid to late 1960s. Housing construction was generally 

underway in the 1970s. The end result was a ‘little Holland’ extending 500 m from 

the line of the coast into the sea.” 

In addition to mine tailings, the area of the hot-spot is near a significant historic foundry, 

and a coastal landfill. It is therefore likely that this reclaimed area also received foundry 

slag or other forms of industrial fill during the long history of Thames. 9 

There are two possibilities about the source of local mercury contamination. Deeper 

sediment samples collected from this area may represent the physical tail of the 

reclamation’s fill material (such as old mine tailings). This tail may extend some 

distance beyond the present-day seawall. Secondly, sediments in this immediate area 

of the Firth of Thames are likely to be receiving leachate from the base of the 

Moanataiari reclamation, as rainwater and groundwater enter and flow through it. 

Three additional threads of evidence are available which either support the case that 

this general area is a localised hotspot, or suggest that the fill material is having an 

impact on concentrations of some trace elements in nearby sediments of the Firth. 

1. Vertical core sample from Kuranui Bay, collected as part of this work. Results from 

analysis of sediments from different depths in this core reveals a strong increase in 

8 Mullock is defined as rubbish, refuse or dirt. 

9 Under the umbrella of current legislation, foundry slag is deposited in an authorised landfill. 

Page 32 Doc # 1120743

concentrations of arsenic, cadmium, mercury and zinc with depth (Appendix 2). The 

deepest sample (25 -30 cm) was found to contain 149 mg/kg arsenic, 2.07 mg/kg 

cadmium, 5.14 mg/kg mercury and 469 mg/kg zinc (Appendix 2). This contrasts 

with the results for vertical core samples from Waiomu Bay and Te Puru. Such a 

relationship, where the oldest sediments contain the highest trace element 

concentrations, suggests that the contamination at Kuranui Bay is primarily a result 

of historic filling and reclamation. It is not yet clear whether the highest result at 

depth may represent the depth where leachate in groundwater has the strongest 

impact on sediment quality, or represents the fill material itself. It might be expected 

that the fill will extend beyond the base of the seawall (Figure 3.2). 

2. Results from Environment Waikato’s 2003 shallow sediment sampling. As 

discussed in Section 3.3.1, shallow (0-2 cm) sediment samples have previously 

been collected from five locations in the Firth of Thames and analysed for 33 

elements (results are provided in Appendix 6). One of these locations was Kuranui 

Bay. A statistical comparison can be made between the results from Kuranui Bay 

composite with those from the other four sampling sites in the same data set. 

Concentrations of 12 of the 33 elements are statistically higher in the Kuranui Bay 

sample (Table 3-9). Of the elements which are present at significantly greater 

concentrations in the Kuranui Bay sample, concentrations of cadmium, silver, 

mercury, zinc, manganese and antimony are substantially enriched, by factors of 

three or more. In addition to these, arsenic is probably enriched in the Kuranui Bay 

sample (at confidence level of just under 95%), by a factor of two. For the elements 

in common with those assayed in the URS sampling round (2005), relative 

enrichments are consistent (compare Table 3-8 with Table 3-9) despite the different 

sampling depths (0-2 cm in the 2003 samples and 0-10 cm in the 2005 samples). 

Cadmium and mercury are the most enriched elements of those in common 

between the two surveys. 

Table 3-9 Trace element concentrations in shallow (0-2 cm) sediment samples from 

Kuranui Bay which are statistically higher than those at four other sites in 

the Firth of Thames at a 95% confidence level. 

Element Measured concentrations (mg/kg dry weight) 

Enrichment 

ratio 

Four non-Kuranui Bay sites 

Confidence 

Kuranui Bay 

Mean interval (Composite result) 

Lower Upper 

Cadmium 0.035 0.010 0.060 0.34 9.7 

Silver 0.067 0.030 0.100 0.30 4.5 

Mercury 0.198 0 0.520 0.84 4.2 

Zinc 53.1 28.9 77.3 199 3.7 

Manganese 474 127 820 1480 3.1 

Antimony 0.26 0 0.64 0.81 3.1 

Molybdenum 0.47 0 1.02 1.22 2.6 

Lead 19.7 7.3 32.2 42.9 2.2 

Caesium 0.74 0.48 0.99 1.2 1.6 

Lithium 12.4 7.5 17.3 18.7 1.5 

Tin 0.40 0.27 0.53 0.6 1.5 

Boron 10.4 8.1 12.7 13.0 1.3 

3. Sediment sampling carried out in the same area in 2004, as part of a resource 

consent for closure of the Thames Landfill (Kingett Mitchell Ltd., 2004). In this work, 

sediment samples were collected from 16 locations around the coast of the 

Moanataiari reclamation (Figure 3-2) to a depth of 14 cm. Of these samples, three 

were in Kuranui Bay, four were seaward of the Thames landfill (Figure 3-2), six 

were south of the landfill, and three were southwest of the landfill. In this case there 

is no specific control group of samples also collected to a depth of 14 cm elsewhere 

in the Firth of Thames from which a statistical comparison might be made. 

Doc # 1120743 Page 33

However, results for mercury show evidence of substantial local enrichment, and 

can be directly compared with sediment quality guidelines (Section 2.5). This 

comparison is made in Table 3-10. 

Table 3-10 Concentrations of mercury in 0-14 cm sediment samples collected from 

the vicinity of the Moanataiari reclamation, and comparison to ANZECC 

(2000) guideline values for sediment quality. Data source: Kingett Mitchell 

Ltd, 2004. 

Mercury concentrations in sediment 

samples (mg/kg dry weight) 

Average mercury concentration 

(mg/kg) & proportion of the 

ANZECC ISQG-High of 1 mg/kg (a 

ratio) 

Factor by which average mercury 

concentration exceeds the 

ANZECC ISQG-Low of 0.15 mg/kg 

Kuranui 

Bay 

2.67; 2.55; 

2.37 

Seaward of 

landfill 

1.47; 1.29; 

1.91; 1.73 

Location 

South of 

landfill 

0.59; 0.35; 

0.86; 0.60; 

1.31; 0.6 

Southwest 

of landfill 

1.02; 0.82; 

2.99 

2.53 1.56 0.60 1.61 

16.9 10.4 4.0 10.7 

This data shows that samples collected from the vicinity of the reclamation by Kingett 

Mitchell in 2004 were strongly enriched with mercury, to a point where sediment in this 

area would meet a reasonable test of the Resource Management Act and Waikato 

Regional Plan definitions of ‘contaminated land,’ on the basis of its mercury content. 

Risks associated with this contamination would be primarily to organisms living in the 

marine sediment. Currently there is no evidence of presence of risks to residents of the 

Moanataiari subdivision, due to the likelihood of a deep capping layer of clean 

overburden having been placed over the older mine tailings or other fill layers. As part 

of a separate project on contaminants in urban soils, a single composite soil sample 

(consisting of 16 x 0-10 cm subsamples) was collected from Moanataiari school in 

January 2007, and was found to contain only normal background concentrations of 

arsenic, cadmium, copper, chromium, lead, mercury, nickel, and zinc. 

Further work is needed to determine the contaminant loadings and environmental 

significance of fill that was used in the Moanataiari reclamation on sediment quality 

nearby in the Firth of Thames, and quantify its significance in relation to other sources 

of contaminants. 

3.5.5 Agriculturally proximate versus control sites 

Sediment samples were collected around the mouth of the Waihou and Piako Rivers to 

determine if agricultural discharges were having an adverse impact on sediment quality 

in the lower Firth of Thames. For this analysis, the data was divided into six composite 

samples representing areas that might be most influenced by agricultural sources – 

these were the Waihou and Piako River mouths and the Opani mudflat between them – 

and 52 other composites for comparison. 

Samples for the Thames stormwater pipeline site and Kuranui Bay were removed from 

the comparison data set, because these sites show evidence of atypical localised 

enrichment (Section 3.5.4). Samples deeper than 10 cm were also excluded from the 

comparison set, and four non-detections were set equal to half their detection limits. 

Results of Student’s t-tests are shown in Table 3-11. 

Page 34 Doc # 1120743

Table 3-11 Results of Student’s t-tests between sites closest to agricultural sources 

and other areas. Mean values are in mg/kg (dry weight), and have not been 

normalised to any other variable. 


Average of sites closest 

to agricultural sources 8.27 0.178 26.6 10.1 0.275 8.70 28.4 98.8 

Average of other sites 22.4 0.072 21.9 19.2 0.166 8.06 23.2 74.4 

Pooled t-test for 

agricultural > other: 

Agricultural significantly 

greater No Yes Yes No No No No Yes 

Probability value of 

pooled t-test; p= 1.0 0.0002 0.0059 1.00 0.11 0.09 0.11 0.0006 

Ratio of averages 

(agricultural/other) where 

difference is significant 2.5 1.2 1.3 

Pooled Student’s t-tests indicate highly significant differences for cadmium, zinc and 

chromium, with sediments of sites closest to agricultural sources containing the highest 

concentrations of these three elements (Table 3-11). The fact that the same sites show 

no statistical elevation of arsenic, copper, mercury, nickel and lead (Table 3-6) 

suggests that the additional cadmium, zinc and chromium in sediments at these 

locations is less likely to be caused by the ongoing influence of historic mining 

discharges than the modern influence of agricultural sources. This idea is supported by 

the fact that many agricultural soils receive high annual loadings of both cadmium (from 

superphosphate fertiliser) and zinc (from facial eczema remedies), but overall, do not 

receive comparably high loadings of arsenic, copper, mercury, nickel or lead (Section 

1.1.3). 

• Relative to its background concentrations, the most enriched element of these 

three is cadmium, concentrations of which are 2.5 times higher at those sites 

closest to agricultural sources than other sites. In terms of total loadings, the most 

enriched is zinc, with the agriculturally influenced sediments containing 24.4 mg/kg 

more zinc than the other areas. 

This statistical analysis suggests that agricultural activities may also contribute 

chromium to receiving environments. Additional loadings and concentrations of 

chromium are less substantial than those of cadmium or zinc (Table 3-11), and the 

source of additional chromium is not known. However, agricultural sources are not 

excluded, because the data of Franklin et al. (2005) indicates that phosphate fertiliser 

and certain N-P-K fertiliser blends can contain high concentrations of chromium. 


In Section 3.5, a range of statistical comparisons have been made between the various 

types of sites that were sampled in the Firth of Thames. Results suggest the following: 

• Data gathered to date provides little evidence that former mining sites along the 

lower eastern coast of the Firth of Thames had, or continue to have, a substantive 

impact on the quality of their nearest coastal sediments, relative to the more 

general impact from other sources (Section 3.5.2). 

• An apparent trend in cadmium and zinc enrichment is evident, with concentrations 

highest at sites closest to the mouths of the Waihou and Piako Rivers (Section 

3.5.2). Student’s t-tests indicate that sediments from the sites closest to agricultural 

sources contain more cadmium and zinc than other areas (Section 3.5.5). 

• There is tentative evidence that a significant proportion of arsenic and copper in 

Firth of Thames sediments may be sourced from the land through weathering and 

erosion processes (Section 3.5.3). 

Doc # 1120743 Page 35

• A site in the vicinity of Thames shows evidence of being a localised hotspot for 

mercury and cadmium (in particular), and zinc in the marine sediments (Section 

3.5.4). This is from Kuranui Bay south to an area adjacent to the Thames 

stormwater discharge pipeline. Results of depth samples at Kuranui Bay suggest 

that the worst contamination was historic. Current evidence suggests that the most 

likely cause is mine tailings and industrial fill that were used as the base of the 

Moanataiari reclamation. The area may warrant closer investigation to evaluate 

contaminant loadings, potential to discharge and environmental significance of any 

industrially-sourced fill that was used in this area. 

• Elevated arsenic, copper, lead, cadmium and zinc concentrations along the lower 

eastern coast of the Firth of Thames (Section 3.3) appear to be dominated by 

larger-scale sources, which are capable of causing an impact over the area as a 

whole. The three most likely larger-scale sources are enhanced weathering and 

erosion following land clearance, the impact of historic mining operations which 

involved disposal of large volumes of tailings directly to the Ohinemuri River 

(Section 1.1.2), and agricultural inputs (Section 1.1.3). 

3.6 Correlations and spatial trends 

3.6.1 Approach and correlation matrix 

Pearson’s correlation coefficients were determined for element concentrations in 

sediments collected as part of this work. Interpretation of the correlations is provided in 

the subsequent sections. 

As previously, samples for the Thames pipeline site and Kuranui Bay were removed 

from the data set before deriving correlation coefficients, because of the evidence of 

strong localised enrichment from a common source in these two areas (Section 3.5.4). 

Data sets for cadmium, chromium, copper, mercury, lead and zinc were log-normalised 

prior to derivation of the correlation matrix. Data sets for arsenic and nickel did not 

require log-normalisation. 

The inter-element correlation matrix is provided in Table 3-12. 

Table 3-12 Pearson’s correlation coefficients for relationships between element 

concentrations in sediments (N = 62 pairs). Yellow shaded boxes represent 

highly significant relationships with probability values of p0.474) 

and pR>0.408). (See text for information about data set 

coverage.) 

[As] 1 

[As] Log[Cd] Log[Cr] Log[Cu] Log[Hg] [Ni] Log[Pb] Log[Zn] 

Log[Cd] –0.255 1 

Log[Cr] –0.232 0.063 1 

Log[Cu] 0.672 0.119 -0.073 1 

Log[Hg] –0.479 0.412 0.157 – 0.485 1 

[Ni] –0.098 –0.208 0.859 – 0.183 0.058 1 

Log[Pb] 0.202 0.546 0.260 0.241 –0.046 0.104 1 

Log[Zn] –0.079 0.850 0.081 0.199 0.160 – 0.150 0.690 1 

The relationship between element concentration and straight-line distance from the 

Waihou River mouth (in kilometres) was also examined, using the same data set but 

without any non-coastal sites (i.e. excluding the Thames mudflat, Thames harbour, and 

Piako River mouth samples). Results of this analysis are shown in Table 3-13. 

Page 36 Doc # 1120743

Table 3-13 Pearson’s correlation coefficients between element concentrations at the 

coastal sites and distance from the Waihou River mouth (N = 49 pairs). 

Yellow shaded boxes represent highly significant relationships with probability 

values of p0.527) and pR>0.456). (See text for 

information about data set coverage.) 

Element Pearson’s correlation 

coefficient (R) 

Arsenic, As 0.454 

Log cadmium, Cd -0.646 

Log chromium, Cr 0.001 

Log copper, Cu -0.080 

Log mercury, Hg -0.310 

Nickel, Ni 0.348 

Log lead, Pb -0.245 

Log zinc, Zn -0.709 

The proportion of fine particles in sediment can influence trace element concentrations. 

Fine particles have a larger surface area per unit weight than course particles and can 

therefore adsorb proportionately more of a given trace element. Measurement of the 

percentage fines was carried out on only 15 samples, across the range of sampling 

locations (Appendix 2). An assumption could be made that the percentage fines 

measured at each location is representative of that location, and this would allow this 

measured fines value to be assigned to all samples from that area. However, the 

validity of this assumption cannot be tested, so it is safer in this case to restrict the 

correlation analysis to the 15 sample pairs. Correlation coefficients between 

percentage fines and element concentrations in the same samples are provided in 

Table 3-14. 

Table 3-14 Pearson’s correlation coefficients between element concentration and 

percentage of fines in the sediment (N = 15 pairs). Yellow shaded boxes 

represent highly significant relationships with probability values of p0.760) and pR>0.641). 

3.6.2 Zinc, cadmium, and lead 

Variable Pearson’s correlation 

coefficient (R) 

Distance -0.747 

Arsenic, As -0.730 

Log cadmium, Cd 0.705 

Log chromium, Cr 0.323 

Log copper, Cu -0.636 

Log mercury, Hg 0.383 

Nickel, Ni 0.405 

Log lead, Pb 0.327 

Log zinc, Zn 0.709 

Concentrations of zinc, cadmium and lead are very highly (p

log (cadmium concentration, mg/kg) 

-0.4 

-0.6 

-0.8 

-1.0 

-1.2 

-1.4 

-1.6 

-1.8 

1.7 1.8 1.9 2.0 2.1 2.2 

log (zinc concentration, mg/kg) 

Figure 3-3 Relationship between zinc and cadmium concentrations (mg/kg) in Firth 

of Thames sediment samples. (See Section 3.6.1 for information about data 

set coverage.) 

log (zinc concentration, mg/kg) 

2.2 

2.1 

2.0 

1.9 

1.8 

1.7 

0 5 10 15 20 25 

Distance (km) 

Figure 3-4 Relationship between concentrations of zinc in sediments and distance 

from the Waihou River mouth in kilometres. (See Section 3.6.1 for 

information about data set coverage.) 

These differences in behaviour suggest the presence of two significant sources of zinc 

and cadmium, one of which is (or was) also a source of lead. In other words, the 

correlations suggest the presence of one significant source of all three metals (lead, 

cadmium and zinc) and another significant source of two metals only (cadmium and 

zinc). Possible sources in each category could include mining, and agriculture, 

respectively. 

Both lead sulphide (galena) and zinc sulphide (sphalerite, which also contains a 

significant quality of cadmium) co-occur naturally in many epithermal mineral deposits 

in the Coromandel and Hauraki goldfields. This natural co-occurrence of these two 

metals could possibility explain the apparent correlation between lead and 

zinc/cadmium. A majority of the base metals sulphide deposits occur around or south 

of Thames (an exception being Monowai Mine on the Paroquet Stream at Waiomu). 

Page 38 Doc # 1120743

In the fifteen samples where percentage fines in the sediment was measured, zinc and 

cadmium are also positively correlated with this variable (Table 3-14). The data also 

shows a decrease in the proportion of fines heading north. This suggests that an 

unknown proportion of the decrease in zinc and cadmium concentrations with distance 

from the Waihou River mouth may be caused by a decreasing content of fines. 

However, this result should be treated with caution, because not all elements show the 

same behaviour – for example, lead, nickel and chromium show no significant variation 

with percentage fines or distance (Table 3-14). In addition, mining itself is likely to have 

been a major source of fines, and it may be artificial to attempt to distinguish fines from 

element concentrations if both were caused by mining. 

Geographical trends in zinc and cadmium concentrations that were detected using 

Pearson’s correlation coefficients (Table 3-13 and Figure 3-3) are in keeping with those 

which became evident using a Student’s t-test approach (Table 3-7 and Figure 3-1), 

where mining control sites were compared to other sites. The two statistical 

approaches are different ways of approaching the same question. 

3.6.3 Arsenic and copper 

Copper and arsenic are also highly (p

presence of natural minerals in the sediments that are enriched in arsenic and copper. 

Another possibility is that arsenic has more effectively spread throughout the Firth of 

Thames than other elements sourced from mining, but this idea is undercut by the 

correlative relationship between concentrations of arsenic and copper. Unlike arsenic, 

where released, copper is expected to be relatively immobile. 

Three factors are consistent with an hypothesis that additional arsenic and copper in 

the Firth of Thames sediments is most likely to be mainly from weathering of minerals 

in coastal areas of the Coromandel Peninsula. 

1. In Section 3.5.3 it was shown that arsenic and copper concentrations are likely to 

be slightly higher in samples collected from nearest the land (landward samples), 

compared with samples collected from nearest the Firth of Thames water (seaward 

samples). 

2. The negative correlation with fines (Table 3-14) suggests that the larger particles 

contribute a greater mass of arsenic or copper to the concentration measurement. 

This behaviour is opposite to that observed for metal enrichment in sediments from 

most industrial sources, where the highest concentrations are commonly 

associated with the finest size fractions because these have the greatest surface 

areas available for metal re-adsorption from solution. However, it is not an 

unexpected behaviour in cases of natural mineralisation. Such a trend implies that 

movement of whole mineral particles may dominate the mass flow of arsenic and 

copper being washed into the Firth of Thames through weathering and erosion 

(rather than dissolution and leaching of arsenic and copper into the aqueous 

phase). 

3. A mineral source would also explain an association between copper, arsenic and 

iron, given that the Coromandel range contains a significant natural abundance of 

volcanic–hydrothermal minerals such as arsenopyrite (FeAsS) and chalcopyrite 

CuFeS2 (Section 1.1.1). Arsenic is a common trace component found in pyrite 

which occurs throughout the Hauraki Goldfield in both ore rock and the host rock. 

Copper is a major element found in chalcopyrite which co-occurs with pyrite, to a 

greater or lesser degree, with pyrite within the Coromandel and Hauraki goldfields. 

However, the data does not rule out a past contribution to arsenic in sediments from 

mining. If the elevated arsenic were from an Ohinemuri mining source, some features 

of the observed pattern might be achieved through one of three mechanisms: 

• The mining contribution did cause elevated arsenic as well as other elements in the 

Firth of Thames, but the arsenic spread more effectively than most of the other 

elements. This is possible and likely for dissolved arsenic, but would not explain the 

correlation with copper, which would be less mobile if it were in the dissolved 

phase. This explanation is therefore unlikely. 

• Dissolved arsenic that entered the Firth during the mining era was flushed out to 

sea, and not much copper made it to the Firth in the first place. Zn-Cd-Pb and Hg 

were transported to the mouth of the Wiahou River, but salted out on entry into the 

Firth of Thames. Under this scenario the modern picture may be a mix of the 

residue of mining for Zn-Cd-Pb and Hg, and local sources for arsenic and copper. 

• The Ohinemuri mining contribution did cause elevated arsenic in Firth sediments, 

but those most highly contaminated sediments are now usually buried deeper than 

10 cm, and any remaining trends with distance (Zn-Cd-Pb-Hg) reflect a residual 

ongoing contribution from the Waihou River. 

Overall, the correlation between arsenic and copper (and both to some extent with iron) 

in sediments, the inverse relationship of both elements with fines, the lack of a 

convincing trend in concentration of either element with distance north, and the 

Page 40 Doc # 1120743

apparent concentration gradient in moving from land to shore, suggest that most of the 

additional copper and arsenic in the Firth of Thames surface sediments (relative to 

Raglan Harbour – Section 3.3.1) may be caused by weathering and erosion of natural 

sulphide minerals in the Coromandel area. As discussed in Section 1.1.1, it is probable 

that such weathering and erosion has been accelerated by land clearance which 

accompanied progressive waves of human colonisation. 

Although the idea that arsenic concentrations in modern surface sediments are 

dominated by local natural sources more readily accounts for observed correlations 

and trends, an influence of past mining operations cannot be ruled out. 

3.6.4 Chromium and nickel 

Chromium and nickel do not appear to be elevated in the Firth of Thames relative to 

Raglan, do not change with distance along the coast, and closely correlate with each 

other (Table 3-12). Concentrations of chromium and nickel in the Firth of Thames’ 

surface sediments appear to be at their natural baseline levels. 

3.6.5 Mercury 

In concentration units (mg/kg), mercury is the most enriched of the elements measured 

in coastal sediments of the eastern coast of the lower Firth of Thames (Section 3.3.1). 

Despite this, correlation data for mercury provides little immediate indication of 

probable source for the additional mercury. 

• Mercury concentrations are negatively correlated with concentrations of arsenic and 

copper (mercury concentrations tend to be higher in samples with less arsenic or 

copper), but are positively correlated with cadmium. 11 Mercury is uncorrelated with 

nickel, chromium, lead or zinc. There is no evidence that mercury concentrations 

are higher in seaward than landward samples (Section 3.5.3), or that mercury 

concentrations are highest in samples collected from nearest the mouth of the 

Waihou River (Table 3-13), or are influenced by fines (Table 3-14). 

On the other hand, there is evidence of strong localised enrichment by mercury in the 

Kuranui Bay and Thames areas, including the Thames mudflats (Table 3-8). The 

correlation statistics discussed above are based on the data set with samples from 

Kuranui Bay and Thames stormwater pipeline area excluded, but the Thames mudflat 

samples retained. 

Closer examination of the data for mercury (Appendix 2) reveals a feature that was 

missed in the statistical overview, and not included in the effect of distance assessment 

(Table 3-13). This is that samples collected from the Piako River mouth appear to be 

unexpectedly elevated in mercury, despite the fact that the catchment of this river is 

agricultural with no known mining influence. Average concentrations of mercury in 

surface samples are presented in Table 3-15 and Figure 3-6. 

11 

These statistics are based on the data set with samples from Kuranui Bay and Thames stormwater pipeline area 

excluded. See Section 3.6.1. 

Doc # 1120743 Page 41

Table 3-15 Average mercury concentrations in surface (0-10 cm or grab) sediment 

samples from all sites. Sites in the vicinity of the Thames pipeline and Kuranui 

Bay are separated out due to evidence of a localised source of mercury, 

cadmium and zinc in this area (Section 3.5.4). One outlier of 1.52 mg/kg 

mercury for Thames mudflat sample SDB584 has been removed from the data 

set prior to calculating these averages. 

Average mercury concentration (mg/kg) 

0.35 

0.30 

0.25 

0.20 

0.15 

0.10 

0.05 

0.00 

Location 

Thames pipeline/Kuranui anomaly 

Mercury concentration 

(mg/kg dry weight) 

Thames stormwater pipeline 0.40 

Kuranui Bay 0.41 

Thames stormwater and wharf 0.28 

Other sites (south to north) 

Piako River Mouth 0.32 

Opani mudflat 0.27 

Thames mudflats and harbour 0.23 

Waihou River mouth 0.22 

Tararu Stream 0.18 

Thornton 0.08 

Te Puru 0.09 

Waiomu Bay 0.08 

Tapu 0.12 

Te Mata 0.15 


Opani mudflat 

Thames mudflats & harbour 


Sampling location 

Tararu 

Thornton 

Te Puru 

Waiomu Bay 

Tapu 

Te Mata 

Figure 3-6 Average concentrations of mercury moving from the Piako River mouth 

east and then north, excluding the Thames pipeline/Kuranui Bay anomaly. 

(Other details are as for Table 3-15.) 

Pooled Student’s t-tests show significantly more mercury (p

is no positive correlation between mercury and elements that may be sourced from 

erosion of the Coromandel ranges (copper and arsenic). However, there is evidence 

that the Piako River may be a significant conduit by which mercury enters the Firth of 

Thames (Table 3-15 and Figure 3-6). Yet, unlike cadmium and zinc, there are no 

particularly substantive sources of mercury in agriculture (Section 1.1.3) and no 

evidence of mercury enrichment in sediments of Raglan Harbour, which is itself 

surrounded by agricultural land (Section 3.3.1). 

Together these observations suggest the presence of a unique source of mercury to 

the Firth of Thames, which is not related to mining, erosion of Coromandel minerals, or 

treatments used in agriculture, but which is delivered to the Firth of Thames via the 

Piako River (potentially among other routes). 

Recent work in mercury chemistry provides a likely reason for this. Wetlands can 

substantially increase the flux of mercury to nearby receiving environments systems 

(O’Driscoll et al, 2005). This is significant because the Piako River passes directly 

through the middle of New Zealand’s largest area of wetlands and peatland (Figure 2.1 

and Appendix 8) and is the main river draining the Hauraki Plains. Water of the Piako 

River can carry sufficient dissolved organic matter and iron to impart a distinct colour. 

Mercury concentrations in the Piako river channel grab sample sediments are about 

four times greater than might otherwise be expected (Table 3-15). Excluding the 

Kuranui Bay anomaly, mercury concentrations in shallow surface sediments (0-2 cm) 

are about seven times higher than those in Raglan harbour (Section 3.3.1). The total 

area of the Piako catchment (Appendix 8) is approximately 1488 km 2 . The area of 

major wetlands within this catchment (Figure 2.1) is approximately 108 km 2 , or about 

7% of the total Piako catchment area. A back-of-envelope calculation suggests that 

mercury enrichment in sediments of the Firth of Thames might be consistent with the 

Hauraki wetlands/peatlands causing a localised increase in mercury flux of about 70 

times. 12 Although this is a very rough estimate and carries a number of untested 

assumptions, such a magnitude is consistent with O’Driscoll et al (2005), who note that 

in the Experimental Lakes Area reservoir project in Ontario, it has been found that 

wetland areas of catchment provide 26-79 times more methylmercury per unit area to 

downstream water than areas that contain no wetlands. In principle, contributions from 

the Piako catchment’s wetlands (Figure 2.1) could, therefore, be capable (in theory) of 

causing most of the observed mercury enrichments in the Firth of Thames sediments. 13 

Mercury mobilised from wetlands to receiving environments is known to be mainly 

associated with dissolved organic matter. As concentrations of dissolved organic 

carbon in drainage waters increase, more mercury is transported. Peatlands would not 

necessarily contain more mercury than elsewhere, but dissolved organic carbon 

formed by oxidation and drainage of peat is likely to be a vehicle for enhanced mercury 

transport to downstream receiving environments. In conjunction with the results of this 

work (e.g. Figure 3.6), it is therefore thought probable that the peatlands of the Hauraki 

Plains contribute a substantial flux of mercury to the Piako River, and this becomes a 

significant source of mercury to the Firth of Thames. 

Such a source would be consistent with the statistical observations about mercury in 

this study, including the fact that the Firth of Thames sediments appear 7-11 times 

more enriched in mercury than sediments of Raglan Harbour. The catchment of Raglan 

Harbour does not include significant areas of peatland. 

The relationship between mercury and arsenic and copper concentrations is shown in 

Figure 3-7. 

12 

i.e. The wetland area is about 7% of the total Piako catchment. 7% of a 50 times enrichment in the 

wetland zone = an overall increase of 4 times (one end of the observed range), and 7% of a 96 times 

enrichment = 7 times (the other end of the observed range). The average of 50 and 96 is 73. 

13 

Excluding the localised Kuranui Bay/Thames pipeline anomaly, which appears to be associated with 

the Moanataitiri reclamation (Section 3.5.4). 

Doc # 1120743 Page 43

log (mercury concentration, mg/kg) 

-0.2 

-0.4 

-0.6 

-0.8 

-1.0 

-1.2 

-1.4 

5 10 15 20 25 30 35 

Arsenic concentration, mg/kg 

-1.4 

0.8 1.0 1.2 1.4 1.6 

log (copper concentration, mg/kg) 

Figure 3-7 Relationship between concentrations of mercury and arsenic, and 

mercury and copper, in Firth of Thames sediments. Site coverage is as 

described in Table 3.15. 

It is initially unclear why there is a negative correlation between concentrations of 

mercury and those of copper or arsenic (Table 3-12; Figure 3-7), given that 

concentrations of these two elements do not significantly change with distance north. 

However, the observed relationships would be consistent with the idea that sites with 

proportionately more of the Coromandel range-sourced mineral grains (richer in copper 

and arsenic) have proportionately less of their sediment sourced from the Waihou and 

Piako Rivers (sediment which may be richer in mercury). The negative correlation 

between arsenic or copper and fines discussed previously (Table 3-14, Section 3.6.3) 

is also consistent with this interpretation. 

Similarly, the positive correlation between mercury and cadmium (Table 3.12) may also 

reflect the influence of sediments sourced from the Piako River, which services an 

agricultural catchment. This correlation may represent two different sources sharing the 

common transport route. Whereas wetlands and peatlands may be a source of 

mobilised mercury, phosphate fertiliser is the most likely agricultural source of 

cadmium, but both elements would be carried to the Firth of Thames by the same river 

system. 

Correlations between mercury and cadmium, arsenic or copper are all, therefore, 

consistent with an hypothesis that a significant amount of mercury entering the Firth of 

Thames may originate from the Hauraki Plains. 

3.6.6 Principal Components Analysis 

Principal Components Analysis (PCA) was also performed on the same data set as for 

the correlation matrix (Table 3.12, Section 3.6.1). Results are provided in Table 3-16. 

As might be expected, PCA results are consistent with the inter-relationships shown in 

the correlation matrix and with interpretations made in the preceding sections. Ninety 

percent of the data variance in the eight elements was associated with the first four 

factors. 

The meanings of factors generated in PCAs require a certain amount of interpretation, 

and for this reason the following descriptive interpretations should not be viewed as 

definitive. However, interpretations which might be placed on each factor are as 

follows: 

Page 44 Doc # 1120743 

log (mercury concentration, mg/kg) 

-0.2 

-0.4 

-0.6 

-0.8 

-1.0 

-1.2

Table 3-16 Results of Principal Components Analysis on the correlation matrix data 

set. Weightings less than -0.4 or greater than 0.4 are highlighted. 

F1 F2 F3 F4 F5 F6 F7 F8 

As 0.26 -0.76 0.36 0.31 -0.32 0.14 -0.01 -0.09 

Cd -0.92 -0.11 -0.25 0.09 0.17 0.02 -0.20 -0.08 

Cr -0.31 0.51 0.76 0.06 0.17 -0.09 0.10 -0.13 

Cu -0.04 -0.81 0.33 0.29 0.36 -0.11 0.02 0.09 

Hg -0.42 0.60 -0.37 0.54 -0.18 -0.05 0.06 0.05 

Ni -0.04 0.54 0.81 0.04 -0.03 0.16 -0.12 0.11 

Pb -0.74 -0.33 0.35 -0.21 -0.37 -0.21 -0.01 0.05 

Zn -0.91 -0.27 -0.07 -0.12 0.06 0.24 0.15 0.03 

• Factor 1 is a ‘zinc-cadmium-lead-mercury’ group, denoting the sum or positive intercorrelations 

between these elements, and accounts for 32% of the data set’s 

variation. This first factor probably represents the sum of mining and agricultural 

sources, with the mercury component reflecting this element’s mild correlation with 

cadmium (Table 3-12). It was speculated that the mild mercury-cadmium 

relationship may come about because the Piako River may act as a conduit for both 

agricultural cadmium, and wetland/peatland-sourced mercury (Section 3.6.5). 

• Factor 2 is an ‘arsenic-copper and opposing elements’ group, and accounts for 

29% of the data variation. This factor denotes that arsenic and copper follow each 

other, but are both inversely related to mercury, chromium and nickel. In terms of 

sources, this factor could be taken to represent direct contributions from erosion of 

the Coromandel (arsenic and copper), and the proportion of this material in relation 

to sediment sourced from elsewhere (mercury increases as arsenic and copper 

decrease). An inverse relationship between arsenic-copper and either chromium or 

nickel is not evident in the correlation matrix (Table 3-12), but the inverse 

relationship with mercury is. However, a PCA can occasionally detect underlying 

relationships that are masked when inter-element correlations pull in different 

directions. If there is a genuine inverse relationship between arsenic-copper and 

chromium-nickel, it might be for the same reasons as the inverse relationship 

between arsenic-copper and mercury (see Section 3.6.5 and Figure 3.7). 

• Factor 3 groups the two metals nickel and chromium and accounts for 22% of the 

data set’s total variation. This ‘nickel-chromium’ factor could be taken to denote the 

natural state of sediments. These two elements are not enriched and in their natural 

state are correlated with each-other (Table 3-12). 

• Factor 4 is a residual ‘mercury only’ factor, which accounts for 7% of the data 

variation. The existence of a ‘mercury only’ factor might be consistent with a source 

of mercury that is not bound up with the other sources (mining, agricultural 

treatments, or erosional contributions). In Section 3.6.5, it is suggested that the 

Hauraki peatlands and wetlands may be such a unique source. 


The focus in Section 3.6 was on inferences that might be made based on correlations 

between the elements. Results suggest the following: 

• Concentrations of zinc, cadmium and lead are highly correlated with each other. 

However, zinc and cadmium concentrations decrease with distance north of the 

Waihou River mouth, whereas lead concentrations do not. For these three 

elements, correlations, trends and relative enrichments are consistent with the 

hypothesis that: 

- Past mining in the Ohinemuri catchment may have been a significant source of 

lead, zinc and cadmium to coastal sediments of the Firth of Thames. 

Doc # 1120743 Page 45

- Agricultural treatments (facial eczema remedies and phosphate fertilisers) may 

be a significant ongoing source of zinc and cadmium (respectively). 

• Copper and arsenic concentrations are also highly correlated to each other and 

also show associations with iron. These elements are uncorrelated with the other 

elements. Several factors suggest that additional arsenic and copper in the Firth of 

Thames sediments may be primarily from weathering and erosion of minerals in 

coastal areas of the Coromandel Peninsula. 

• Mercury is the most enriched element in coastal sediments of the eastern coast of 

the lower Firth of Thames, in concentration units (mg/kg). However, there is no 

evidence that this additional mercury may be sourced from past mining, erosion of 

natural minerals, or agricultural treatments. Instead, observations suggest the 

presence of a unique source of mercury to the Firth of Thames, which is delivered 

to the Firth of Thames via the Piako River (among other potential conduits). Based 

on recent literature, it is thought possible that a substantial flux of mercury may 

enter the Piako River from the wetlands and peatlands of the Hauraki Plains. 

• Concentrations of chromium and nickel in the Firth of Thames’ surface sediments 

correlate strongly with each other and appear to be at their natural baseline levels. 

Page 46 Doc # 1120743

4 Summary and recommendations 

4.1 Summary 

4.1.1 Relative enrichments 

Arsenic, cadmium, copper, mercury, lead and zinc are enriched in sediments of the 

lower eastern coast of Firth of Thames, relative to concentrations present before 

Polynesian and European colonisation, and reference concentrations in sediments 

from Raglan Harbour. Concentrations of the other five elements measured in some or 

all samples (chromium, nickel, aluminium, iron and lithium) are more typical of those 

observed in clean harbour sediments in other areas. Relative to its expected 

background concentration, the most highly enriched element is mercury. The average 

concentration of mercury in the lower Firth’s sediments is approximately seven times 

higher than in otherwise comparable reference sediments. In absolute mass terms, the 

most highly enriched element is zinc. The Firth’s sediments contain about 10 mg/kg 

more zinc than might otherwise be expected. 

4.1.2 Comparison to guidelines 

The two elements which are nearest to or occasionally exceed guideline values for 

sediments are arsenic and mercury. Arsenic and mercury concentrations in the Firth of 

Thames coastal sediments are not extremely high, but are at a point where they may 

have adverse effects on some sediment-dwelling organisms, at some locations. Both 

arsenic and mercury exceeded their respective ISQG-Low guidelines at eight of the 

thirteen (62% of) sites sampled by URS NZ Ltd. These were not the same sites. Sites 

with the highest mercury were not the sites with the highest arsenic. Although 

concentrations of copper, cadmium, lead and zinc were higher than typical values for 

uncontaminated sediments, they are still well below the lowest sediment quality 

guideline values. These elements are unlikely to pose a risk to the health of the Firth’s 

aquatic ecosystems. 

4.1.3 Local and general sources 

Only one area (at two adjacent sampling locations) stands out as a hotspot of localised 

metal contamination, which looks likely to have been from mine tailings and 

urban/industrial fill deposited in the area as part of land reclamation. This is the 

sediment in Kuranui Bay and an adjacent area in northern Thames, which appears to 

have become contaminated as a result of the Moanataiari reclamation. Depth results 

suggest that the worst contamination in this area was historic, but an ongoing 

contribution from leachate from the base of the reclamation is also likely. 

Apart from this area, the influence of local sources appears minor. The results provide 

little evidence that former mining sites along the lower eastern coast of the Firth of 

Thames had, or continue to have, a significant impact on the quality of their nearest 

coastal sediments, relative to the more general impact from other sources. The clearest 

evidence of a local effect from past mining is at Waiomu Bay, where sediment samples 

were statistically enriched with seven of the eight trace elements tested. However, 

although (statistically) significant, these enrichments were not particularly substantial. 

Although there is some evidence for the presence of the occasional grain of a metalrich 

mineral in sediments, more generally the data suggests that elevated arsenic, 

copper, lead, cadmium and zinc concentrations along the lower eastern coast of the 

Firth of Thames are likely to be dominated by larger-scale sources, which are capable 

of causing an impact over the area as a whole. The three most likely larger-scale 

sources are enhanced weathering and erosion following land clearance, the impact of 

historic mining operations which involved disposal of large volumes of tailings directly 

to the Ohinemuri River, and agricultural inputs. A fourth possible source identified in 

Doc # 1120743 Page 47

this work (for mercury) may be dissolved organic matter entering the Firth of Thames 

from wetlands and peatlands of the Piako River catchment. 

4.1.4 Probable dominant sources by element 

(Note: Sources of the trace elements discussed in this section are in addition to those 

responsible for the natural background.) 

Zinc and cadmium 

Results for zinc and cadmium are the most consistent with the existence of two 

significant diffuse sources of zinc and cadmium to the Firth of Thames: past mining in 

the Ohinemuri-Waihou catchment and current agricultural treatments. 

• Ohinemuri catchment mining is thought to be the most likely mining-related source 

of zinc and cadmium, because of the large volumes of tailings involved. The 

Waihou River is known to have received between 500-800 tonnes per annum of 

tailings from the Ohinemuri River for over 50 years. 

• Facial eczema remedies are the most substantial potential source of zinc from 

agriculture, and phosphate fertilisers are the most substantial source of cadmium 

Lead 

Results for lead are consistent with the existence of one significant diffuse source of 

additional lead, which is most likely to be the influence of past mining in the Ohinemuri 

catchment. This mining source appears to have been a source of significant lead, zinc 

and cadmium. Unlike zinc and cadmium, there is no evidence of an additional 

agricultural source of lead. 

Copper and arsenic 

Several factors suggest that additional arsenic and copper in the Firth of Thames 

sediments is primarily from weathering of minerals such as pyrite in coastal areas of 

the Coromandel Peninsula. It is likely that the rate and extent of weathering and 

erosion, and influx of arsenic and copper, was enhanced by historic land clearance 

activities around the coastal areas of the Coromandel Peninsula, following Polynesian 

and European settlement in the area. 

Mercury 

For mercury, the most enriched element, there is no positive evidence that past mining, 

erosion of natural minerals, or agricultural treatments have been significant sources to 

the Firth of Thames sediments. Rather, the results suggest a unique source of mercury 

which is delivered to the Firth of Thames via the Piako River among other potential 

conduits. Based on recent literature, it is thought likely that this mercury may originate 

from drainage of the wetlands and peatlands of the Hauraki Plains. In addition to this, 

the area around the Moanataiari reclamation is a localised hot-spot of mercury 

contamination. 

Page 48 Doc # 1120743

4.2 Recommendations 

In this section, specific recommendations are numbered and italicised. The rationale for 

each recommendation is provided in the non-italicised text immediately preceding the 

recommendation. 

Although several elements are enriched in the Firth of Thames sediments, it is not yet 

clear whether their concentrations are at steady state, increasing or decreasing. In the 

face of a steady source of a given metal, concentrations can sometimes decrease as a 

result in a greater influx of clean sediment causing dilution. Conversely, better control 

of ‘clean’ sediment inputs can result in a relative increase in the concentrations of 

contaminants in deposited sediments. 

1. It is recommended that sediments of the Firth of Thames be sampled once every 

five years to allow for early warning, in the event that concentrations of one or more 

trace elements are gradually increasing. Future testing could also include related 

variables, such as concentrations of dissolved organic carbon in the Piako River. 

In terms of diffuse contamination, two elements stand out as a consequence of 

exceeding sediment quality guidelines at several locations. These are mercury and 

arsenic. There is reasonable evidence that the additional arsenic was caused by 

enhanced weathering and erosion of the Coromandel Peninsula, following colonisation 

and land clearance. There may be little that can be done about this source, but options 

may be worth investigating. Arsenic does not bioaccumulate. For mercury, there is 

tentative evidence that the element is mobilised in drainage water from the peatlands 

and wetlands of the Hauraki Plains. Mercury is of more inherent concern that arsenic, 

due to its ability to change its chemical form and accumulate both in organisms, and up 

food chains (bioaccumulation and biomagnification). This is particularly given that the 

Firth of Thames is an important site for waders and shorebirds, and the intertidal area 

from Kaiaua to the Waihou River mouth is listed as a wetland of international 

importance under the Ramsar Convention. 

2. For mercury, it is recommended that future work focus on: 

a. Quantifying the relative significance of wetlands and peat deposits in the 

Hauraki Plains as sources of mercury to the Firth of Thames, relative to other 

sources, including possible geothermal inputs. A reliable estimate of mercury 

fluxes from various sources would require ultra-trace measurements of mercury 

concentrations in waters and suspended sediments entering the Firth of 

Thames over one or more seasons, and flow volumes. 

b. Identifying land management factors that would increase or decrease mercury 

inputs to the Firth of Thames. 

c. Identifying the most significant risks associated with mercury which has entered 

the Firth of Thames sediment reservoir. This would need to include an 

assessment of likely trends in sediment mercury concentrations, and probable 

chemical transformations and their significance. A focus on the significance of 

mercury in the Firth of Thames to both wildlife, and human food sources, would 

be appropriate. 

d. Mangrove colonisation has the potential to substantially alter mercury chemistry 

in colonised areas. Ideally, the relationships between mangrove establishment 

and mercury cycling (including mobilisation and fixation) should also be 

investigated. 

3. For arsenic, investigate the feasibility of any simple erosion control measures that 

could be brought to bear. Measures to limit erosion should also reduce the influx of 

Doc # 1120743 Page 49

arsenic (and copper), and should eventually result in a lower concentration of 

arsenic in surface sediments. 

One area stands out as a localised hotspot of contamination by several elements, most 

notably mercury. This is Kuranui Bay and an adjacent part of northern Thames, in the 

area of the pipeline. Sediments in this area appear to have become contaminated as a 

result of fill material used in the base of the Moanataiari reclamation during the long 

history of Thames. Potential risks that the area may pose to the wider ecosystem have 

not been quantified. Potential ongoing discharges are unknown. Contaminated sites 

investigations are carried out in stages, called tiers. A tier I investigation would include 

a detailed review of history and limited specific sampling. Results of the tier I 

investigation are used to guide the tier II investigation, which typically includes more 

detailed sampling and a site-specific risk assessment. 

4. Specific tier I and II contaminated site investigations should be carried out in the 

area of the of the Moanataiari reclamation to determine hazards, pathways, risks to 

aquatic organisms, and management options. 

Page 50 Doc # 1120743

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Appendix 1. Further details of each location from which 

sediment samples were collected by URS as part of this study. 


The Waihou River is the largest river within the study area. The drainage catchment 

includes the following mining inputs: 

• Mt Te Aroha (Tui Mine) – a base metal mine which operated between 1967 and 

1973 and which is regarded as one of New Zealand’s most contaminated sites. 

• Karangahake Gorge, sections of which were mined between 1882 and1933. 

• Martha Hill mine in Waihi, which is currently operational, but which operates 

under modern resource consent conditions which minimise potential impacts. 

Historical evidence suggests that tailings from several different mines containing high 

concentrations of pyrite (FeS2) and other metal sulphides, were deposited directly into 

the Waihou River and its tributaries during the early 1900s. Water and sediment 

sampling conducted in the catchment has previously indicated significant heavy metal 

contamination (Livingston, 1987). 

In addition to possible metal contamination due to historical mining operations, 

agricultural activities may result in elevated cadmium concentrations in sediments. 

Urban inputs from Waihi, Paeroa and Te Ahora townships also drain to the Waihou 

River. Two composite samples were collected from this area, with each consisting of 

five sub-samples (Appendix 1). Sampling was conducted on the western and eastern 

banks of the river mouth using an Eckman grab sampling device deployed from a boat. 

Sub-samples collected and accepted using this method were surface samples, to a 

nominal depth of at least 10 cm (see Section 2.3.3 for further details). Several of the 

sub-samples were collected by URS staff on foot, as the fast-flowing current in the 

Waihou River prevented sampling using the Eckman bottom grabber. 

Piako River mouth and Opani mudflats 

There is no known mining influence in the Piako River catchment area, but agricultural 

inputs are a potential source of trace elements in this area. Three composite sediment 

samples were collected from this area, each consisting of five sub-samples (Appendix 

1). Piako River mouth sampling was conducted on the western and eastern banks of 

the river mouth. Sites were accessed by boat on an incoming tide and the sub-samples 

collected with an Eckman bottom grab sampler (Section 2.3.3) to a nominal depth of 10 

cm. Samples could not be collected from the edge of the channel beyond the location 

of SDB 578, as the sediment was too hard to penetrate. Sediment samples that were 

collected from the main channel contained mainly shell fragments and could not be 

analysed. A separate composite (made of five sub-samples of 0-10 cm depth) was 

collected from the Opani mudflat (see point SDB568 in Figure 2-2), which is situated 

between the mouths of the Waihou and Piako rivers (Appendix 1). 

Thames urban samples 

Discharges of stormwater from urban areas can result in elevated concentrations of 

copper, lead and zinc in sediments. Two general sampling locations near the Thames 

township were used as part of this sampling programme. 

Stormwater pipeline vicinity 

Three composite samples (each made of five sub-samples of 0-10 cm depth) and one 

vertical core consisting of two depths (0-2 cm and 2-10 cm) were collected from the 

area of the Thames stormwater pipeline (Appendix 1). Sub-samples were collected 

using a plastic hand trowel to a depth of 10 cm (i.e. 0-10 cm, Section 2.3.1), and 

vertical profile samples were collected with a sediment corer. One composite sample 

(consisting of five sub-samples) was taken along the walkable length of the pipeline. 

Two of the sub-samples for the other composite were collected along the channel of a 

small stream, which discharged from a small PVC pipe (approx. 50 mm OD) 

Doc # 1120743 Page 53

immediately adjacent to the pipeline outfall. Sediments were generally very fine-grained 

silty sediments, with the upper 2-3 cm centimetres appearing to be an oxic layer. 

Thames stormwater outlet and wharf areas 

One composite sediment sample (consisting of five 0-10 cm sub-samples) was 

collected from each of these two areas, using a plastic hand trowel (Appendix 1). Subsamples 

for the stormwater composite sample were collected from the true right-hand 

side of the channel and adjacent to the Thames stormwater outlet. They were collected 

during a storm event. These samples were found to contain very fine-grained silty 

sediments, with the upper 2-3 cm appearing to be an oxic layer. The wharf area is also 

in the vicinity of the Kauaeranga River outflow (NZMS T13 360473). Samples from this 

area were found to contain mainly fine to medium grain silty sediments, with the upper 

2-3 cm appearing to be an oxic layer. 

Thames Harbour mudflats and deeper harbour 

The Thames Harbour mudflats might receive trace element inputs from both distant 

and nearby sources. The Piako River mouth may contribute significant sediment 

carried from the Hauraki Plains. Inputs from the urban area could result in elevated 

concentrations of copper, lead and zinc as a result of road and roof run-off and 

industrial discharges. The Kauaeranga River also discharges into Thames Harbour. 

The Kauaeranga Valley was mined for cinnabar (mercuric sulphide, HgS) between 

1899 to approximately 1906 (Watson, 1989). A total of eight composite sediment 

samples (each made of five sub-samples of 0-10 cm depth) were collected from the 

Thames mudflats (Appendix 1). The mudflats were observed to contain mainly fine to 

medium grain silty sediments, with the upper 2-3 cm appearing to be an oxic layer. Two 

of these samples were analysed in duplicate for quality assurance purposes. A 

separate composite sample (made of five sub-samples of 0-10 cm depth) was collected 

from further out into the Thames harbour (point SDB569 in Figure 2-2) to a nominal 

depth of about 10 cm, using the Eckman grab sampler. 

Tararu Stream 

Historical mining activities were undertaken at several locations within this catchment. 

Access to the site was gained from the shore. Six composite sediment samples (each 

comprising five sub-samples of 0-10 cm depth) were collected from the intertidal zone 

surrounding the stream mouth at this location using the plastic hand trowel (Appendix 

1). These were mainly fine to medium grain gravels containing some medium grain 

sands. Two of these samples were analysed in duplicate for quality assurance 

purposes. 

Tapu Stream 

Historical mining activities have been undertaken at several locations (including 

Sheridan and Mahara) within this catchment between 1869 and 1906. Access to the 

site was gained from the shore. Six composite sediment samples (each comprising five 

sub-samples of 0-10 cm depth) were collected from the intertidal zone surrounding the 

stream mouth using the plastic hand trowel (Appendix 1). Sediments from this site were 

found to consist mainly of coarse grain gravels. 

Te Puru 

This was a control site, as there have not been any significant past mining or 

agricultural activities within the catchment. Access to the site was gained from the 

shore. As with Tararu and Tapu, six composite sediment samples (0-10 cm) were 

collected in the intertidal zone surrounding the stream mouth. In addition, a vertical 

profile sample was collected using a sediment corer, with sub-samples from depths 0-2 

cm, 2-10 cm, 15-20 cm and 25-30 cm being analysed individually. Sediments from this 

site were found to consist mainly of medium to coarse grain sands. 

Thornton Bay 

Like Te Puru, this was also a control site, as there have not been any significant past 

mining or agricultural activities within the catchment. Access to the site was gained 

Page 54 Doc # 1120743

from the shore. Six composite sediment samples (each comprising five sub-samples of 

0-10 cm depth) were collected from the intertidal zone surrounding the stream mouth 

using the plastic hand trowel (Appendix 1). Sediments from this site were found to 

consist of mainly medium to coarse grain sands. 

Waiomu Stream 

A number of mines operated within this valley between 1890 and 1936 (including 

Paroquet, Broken Hill, Monowai and Comstock Mines). Access to the site was gained 

from the shore. Exploration in this area from 1971 to 1973 and 1983 to 1984 has 

revealed the presence of ore bodies still within this region (Moore et al., 1996). The ore 

deposit has been described as being an epithermal vein deposit with significant base 

metal sulphides (>3%). These would have the potential to elevate concentrations of 

elements in sediments if the ore body undergoes oxidation. In 1981, a preliminary 

survey of the Waiomu stream conducted by the Department of Scientific and Industrial 

Research (DSIR), National Water and Soil Conservation Authority (NWASCA), the 

Ministry of Agriculture and Fisheries (MAF) and the Hauraki Regional Water Board 

(Livingston, 1987) detected elevated concentrations of various trace elements (in 

particular, arsenic, cadmium and zinc) in the water and sediment of the Comstock 

Stream (a tributary of the Waiomu Stream). Slightly elevated concentrations of copper, 

lead and zinc have been detected in several locations within the Waiomu Stream 

water, and elevated concentrations of lead and zinc have been detected in Waiomu 

Stream sediments (Livingston, 1987). Six composite sediment samples (each with five 

0-10 cm sub-samples) were collected in the intertidal zone surrounding the stream 

mouth. One of these was analysed in duplicate for quality assurance purposes. In 

addition, a vertical profile sample was collected using a sediment corer, with subsamples 

from depths 0-2 cm, 2-10 cm, 15-20 cm and 25-30 cm being analysed 

individually (Appendix 1). Sediments from this site were found to consist of mainly 

coarse grain gravels. 

Te Mata 

This was a third control site, as there have not been any significant past mining or 

agricultural activities within the catchment. Access to the site was gained from the 

shore. Six composite sediment samples (each comprising five sub-samples of 0-10 cm 

depth) were collected from the intertidal zone surrounding the stream mouth using the 

plastic hand trowel (Appendix 1). Sediments from this site were found to consist of 

mainly medium to fine grain sands. 

Kuranui Bay 

This location has been sampled in the past by Environment Waikato and elevated 

mercury and zinc concentrations were detected. No mining activities are known to have 

occurred in this catchment, but there are a number of old mines nearby surrounding the 

Thames township. Access to the site was gained from the shore. Seven composite 

sediment samples (each made of five 0-10 cm sub-samples) were collected in the 

intertidal zone surrounding the stream mouth, with a plastic hand trowel. One of these 

was analysed in duplicate for quality assurance purposes. In addition, a vertical profile 

sample was collected using a sediment corer, with sub-samples from depths 0-2 cm, 2- 

10 cm, 15-20 cm and 25-30 cm being analysed individually (Appendix 1). Sediments 

from this site were found to consist of mainly fine to medium grain silts. All sediment 

samples were collected from the south side of the Pukehinau Stream and SDB 562 

was collected from around the area of a stormwater pipe. 

Doc # 1120743 Page 55

Appendix 2. Measured concentrations of eight trace 

elements in 78 sediment samples collected as part of this 

study. 

Unless otherwise indicated sampling depths are 0-10 cm or surface grab samples (see 

Appendix 1). All element concentrations are in mg/kg (ppm) dry weight. 

Te Mata 

As Cd Cr Cu Hg Ni Pb Zn Li Fe Al 

Near beach SDV790 23.4 0.03 19.9 11.4 0.16 8.4 25.9 60.4 

SDV795 27.3 0.03 21.5 11.3 0.15 9.1 30.6 64.4 

SDV794 27.4 0.03 20.3 11 0.16 8.2 30 62.9 

Far beach SDV791 27.3 0.03 21.5 11.3 0.15 9.1 30.6 64.4 

Tapu 

Page 56 Doc # 1120743 

% 

Fines 

SDV792 26.9 0.03 20.9 11.5 0.16 8.6 30.8 64.3 17 32300 18300 4.81 

SDV793 28.1 0.03 20 11.2 0.15 8.4 31.5 64.1 

Near beach SDV796 23.4 0.02 22.7 13.9 0.11 8.7 9.32 58.2 

SDV801 18.8 0.02 19.2 13.7 0.09 8.1 8.16 53.1 

SDV800 16.9 0.03 19.5 13.7 0.13 8.4 11.1 57.4 

Far beach SDV797 27.3 0.02 18.3 14 0.21 8.2 5.63 51.5 

Waiomu Bay 

SDV798 19.7 0.02 17.8 14.4 0.1 8.4 6.74 52.4 16 25600 12900 1.36 

SDV799 21 0.02 19.8 14.2 0.1 8.2 7.93 53.7 

Near beach SDB611 34.5 0.1 26.1 39.9 0.08 8.9 45 79.2 

SDB613 26.3 0.12 30 34.1 0.06 9.8 37.5 83.3 

Far beach SDB612 34 0.17 30.9 41.6 0.09 8.6 65.3 97.1 17 44900 8830 2.14 

QA/QC 

Duplicate 

(SDB 

594) 25.9 0.1 29.2 39.4 0.07 9.8 29.7 84.1 

SDB614 29.6 0.1 26.9 35.2 0.16 9.4 30.7 82.6 15 35900 8890 1.11 

Stream SDB 615 21 0.02 20.8 17 0.07 7.9 20.9 71.7 

Depth samples 

Te Puru 

0-2 cm SDB619A 21

Appendix 2 continued... 

Thornton 


Near beach SDV802 21.9 0.03 20.6 17.2 0.07 8.2 22.7 72.1 

SDV803 24.4 0.03 22 19.3 0.08 7.8 20 70.2 

SDV804 24 0.03 20.6 18.3 0.07 8 19.6 69.4 

Far beach SDB615 21 0.02 20.8 17 0.07 7.9 20.9 71.7 

Tararu 

Stream 

Doc # 1120743 Page 57 

% 


SDB616 24.1 0.03 22.8 17.4 0.08 8.6 26.3 78.6 20 32400 11000 2.35 

SDB621 22.7 0.04 22.1 17.6 0.09 8.3 23.6 74 

Near beach SDB602 20.5 0.12 26.7 14 0.34 9 30.5 100 31 37500 23400 86.6 

SDB601 23.1 0.18 14.2 23.3 0.15 5 24.5 93.8 

SDB600 30.4 0.07 13.9 23.8 0.15 5.5 25.4 87.6 

Far beach SDB597 21.1 0.16 13.5 20.9 0.18 5.7 25.5 80.3 

Kuranui 

Bay 

SDB598 25.9 0.1 15.3 25.7 0.15 5.8 24.4 92 18 40800 12500 2.99 

QA/QC 

Duplicate 

(SDB 595) 26.3 0.21 15.8 31.3 0.14 5.6 24.1 104 

SDB599 24.4 0.32 14.8 22.3 0.14 5.7 24.8 136 

Near beach SDB563 20.7 0.34 22.4 15.8 0.36 7.6 28.7 130 

SDB560 20.7 0.34 22.4 15.8 0.36 7.6 28.7 130 

SDB558 36.9 0.21 21.8 12.7 1.12 7.6 26.9 102 

QA/QC 

Duplicate 

(SDB 591) 23.4 0.18 25 12.1 0.53 8.6 29.2 102 

Far beach SDB561 73.7 0.69 16.1 22.3 1.43 6.8 30 206 

SDB559 23.4 0.18 25 12.1 0.53 8.6 29.2 102 29 30000 23300 87.2 

SDB562 79.3 0.61 18.2 23.1 1.46 7.5 28.2 181 20 32900 13500 37.7 

Depth 

samples 

0-2 cm SDB812 16.8 0.19 29 12.9 0.35 9.5 33.9 116 

2-10 cm SDB813 30 0.92 24.5 20.5 0.73 9.1 34.5 243 

15-20 cm SDB814 50 0.71 24.6 22.7 0.89 9.5 48.8 222 

25-30 cm SDB815 149 2.07 16.4 44.6 5.14 7.4 60.7 469 17 30300 12300 61.2 

Thames Wharf and stormwater 

Thames mudflats 

Deeper Thames Harbour 

Thames pipeline 

SDB567 8.7 0.1 22.8 10 0.21 7.3 25 80 

SDB602 20.5 0.12 26.7 14 0.34 9 30.5 100 

SDB580 14.2 0.05 26.9 16.2 0.16 8.7 14.3 62.1 

SDB581 15.6 0.08 24.9 16.4 0.19 8.5 15.9 65.9 15 34700 20500 19.1 

SDB582 13.8 0.03 21.9 13.5 0.14 8.8 15.5 56.2 

SDB583 15.1 0.05 23.6 14.2 0.17 8.6 17 59.7 

SDB584 17.1 0.05 28.7 18.5 1.52 8.5 12.3 57.4 12 33200 16600 9.49 

SDB585 14.3 0.06 21.7 14.1 0.15 8.4 16.1 59.7 

SDB576Av 15.5 0.05 27 16.1 0.47 8.6 14.9 63.1 14 34100 15300 4.76 

SDB573Av 12.1 0.12 22.3 10.1 0.22 8 24.9 78.9 21 29800 17000 58.8 

SDB569 9.6 0.23 26.4 11.1 0.34 9 32.5 113 

SDB564 27.1 0.31 26.5 15.8 0.42 9.3 37.1 143 

SDB565 25.7 0.53 26.3 18.5 0.54 9.5 39.5 183 27 31900 23300 94.6 

SDB566 433 0.32 22.9 16.5 0.53 8.5 40 145 23 53400 18900 77.7 

0-2 cm SDB810 16.3 0.32 25 14.3 0.37 8.7 30.7 124 

2-10 cm SDB811 28.5 0.5 19.2 19.3 0.49 7.7 30.7 151

Appendix 2 continued... 



Opani 

mudflat 


SDB574 5.1 0.08 31.4 7.6 0.14 8.4 19 72.3 

Page 58 Doc # 1120743 

% 


SDB575 9.4 0.14 30.1 12.2 0.29 10 34 111 31 33500 26500 95.8 

SDB578 7.5 0.19 21.5 9.3 0.28 7.6 25.7 90.9 

SDB579 10.5 0.27 25.3 10.9 0.33 8.9 30.8 113 22 24800 18800 93.7 

SDB571 9.6 0.22 26.1 10.9 0.34 8.9 31.9 107 26 27500 19600 94 

SDB568 7.5 0.17 25.1 9.7 0.27 8.4 28.9 98.5

Appendix 3. Quality assurance and quality control results 

A. Sampling and measurement precision in relation to site heterogeneity 

Six composited samples were re-composited by the laboratory to evaluate the 

variability in both the sample and the laboratory procedures. To determine the precision 

of the analysis, the relative percentage differences (RPD) is calculated for replicate 

samples via the procedure outlined in section 1020 of the American Public Health 

Association (APHA) standard method. If the RPD is within ± 25% the analysis is 

considered to be acceptable. RPD were calculated for various samples within each 

sample run and compared to one another. Results are provided in Table 1. 

Table 1. Relative percentage difference (RPD) of replicate samples. 


SDB612 34 0.17 30.9 41.6 0.09 8.6 65.3 97.1 

QA/QC SDB 594 (dup. SDB612) 25.9 0.1 29.2 39.4 0.07 9.8 29.7 84.1 

SDV792 27.4 0.03 20.3 11 0.16 8.2 30 62.9 

QA/QC SDB 593 (dup. SVB792) 26.5 0.03 21.4 11.9 0.15 9 31.5 65.6 

SDB558 36.9 0.21 21.8 12.7 1.12 7.6 26.9 102 

QA/QC SDB591 (dup. SDB558) 23.4 0.18 25 12.1 0.53 8.6 29.2 102 

SDB598 25.9 0.1 15.3 25.7 0.15 5.8 24.4 92 

QA/QC SDB 595 (dup. SDB598) 26.3 0.21 15.8 31.3 0.14 5.6 24.1 104 

OZ SDB573B 11.1 0.11 24.4 11.7 0.23 8.4 26.5 83.7 

QA/QC SDD573E 13 0.12 20.1 8.4 0.2 7.5 23.2 74 

SDB580 14.2 0.04 26.1 16 0.12 8.7 14 61.1 

QA/QC SDB590 (dup. SDB580) 14.1 0.05 27.7 16.3 0.19 8.6 14.5 63.1 

Relative percentage difference 

612/594 27.0 51.9 5.7 5.4 25.0 -13.0 74.9 14.3 

792/593 3.3 0.0 -5.3 -7.9 6.5 -9.3 -4.9 -4.2 

558/591 44.8 15.4 -13.7 4.8 71.5 -12.3 -8.2 0.0 

598/595 -1.5 -71.0 -3.2 -19.6 6.9 3.5 1.2 -12.2 

573B/573E -15.8 -8.7 19.3 32.8 14.0 11.3 13.3 12.3 

580/590 0.7 -22.2 -5.9 -1.9 -45.2 1.2 -3.5 -3.2 

The RPD for arsenic, cadmium, mercury and lead in sample SDB612/SDB594 are 

greater than the quality control criteria. The composites of this sample were reanalysed 

to determine if the variability was due to analytical variability, sample 

variability or sample heterogeneity. The lead concentration in sample SDB612 was 

significantly higher than in any other surficial sample collected in this study. To 

determine if result was caused by a single high sub-sample, each of the sub-samples 

was re-analysed. A comparison of the average values of the all of the sub samples with 

the reported values of the composite SDB612 shows that the RDP for cadmium, 

mercury and lead are outside the acceptable range. The high variability of cadmium 

and mercury may be due to low concentrations of those samples present in the 

sediment. Analytical values near the sample detection limit will have a larger signal-tonoise 

ratio and, therefore, a greater RDP between replicates. Several other QA/QC 

samples (SDB598/595 for cadmium and SDB580/590 and SDB558/591 mercury) also 

display high RDP for these compounds which may be due to relatively low 

Doc # 1120743 Page 59

concentrations of these compounds presents in the samples. The variability in the 

arsenic concentration in the sub-samples and the composite are similar, which suggest 

the variability between replicates SDB594 and SDB612 is not due to the compositing or 

sample heterogeneity. The large variability in lead between SDB594 and SDB612 was 

not reproduced in the subsequent re-analysis of the sub-samples (Table 2). It is likely, 

therefore, that this outlier was caused by a small grain of lead minerals or metallic lead 

being present in the sample, which skewed the analytical result. Therefore, the first 

result (the outlier) was not considered to adequately represent the composition of the 

sediment in this location. 

Table 2. Analysis of individual sub-samples of Composite SDB612. 

Sample number As Cd Cr Cu Hg Ni Pb Zn 

SBD612A 31.7 0.08 22.1 35.5 0.06 8.6 33.8 79.9 

SDB612B 34.1 0.1 32.7 39.6 0.05 9.2 31.5 85.8 

SDB612C 41.4 0.12 25.5 35.4 0.07 7.9 32 90.3 

SDB612D 29.8 0.11 24.2 39.8 0.06 8.7 35.3 84.4 

SDB612E 34 0.1 24.5 39.7 0.06 8.2 28.2 81.5 

Average 34.2 0.10 25.8 38.0 0.06 8.5 32.2 84.4 

std dev 4.4 0.01 4.1 2.3 0.01 0.5 2.7 4.0 

95% conf error 5.5 0.02 5.0 2.9 0.01 0.6 3.3 5.0 

%conf error of mean 16.0 18.1 19.5 7.6 14.6 7.2 10.3 6.0 

Value of Composite SDB612 34 0.17 30.9 41.6 0.09 8.6 65.3 97.1 

95% lower 28.7 0.08 20.8 35.1 0.05 7.9 28.8 79.4 

95% upper 39.7 0.12 30.8 40.9 0.07 9.1 35.5 89.4 

RDP 0.6 -50.0 -18.0 -9.0 -40.0 -0.9 -68.0 -14.0 

Values highlighted in red exceed QA/QC criteria. 

Quality control reports 

Five quality control reports cover the analysis undertaken by Hills Laboratories for the 

various sample batches that were analysed. All quality control data contained with the 

reports were acceptable, except for sample batch 382278 (which includes samples 

collected from Te Mata Bay, Tapu, Thorton, Thames Harbour and Waihou River 

mouth). Duplicate samples for lead on sample 382504/4 had greater sample variation 

than would normally have been expected. Hills Laboratories believed that this 

variability was due to heterogeneity of the sample rather than analytical error or crosscontamination. 

This sample was not a sample submitted by URS and was not taken as 

part of this study. 

Page 60 Doc # 1120743

Appendix 4. Metal concentrations in sediments normalised 

against various benchmark variables. 

A. Grain size normalisation: sediment metal concentration divided by 

percentage fines. Normalised trace element concentrations are in mg/kg dry 

weight. 

Location Sample % As Cd Cr Cu Hg Ni Pb Zn 

Waiomu Bay 

number Fines 

Far beach SDB612 2.14% 1589 7.9 1444 1944 4.2 402 3051 4537 

Te Mata 

SDB614 1.11% 2397 9.0 2424 3171 14. 

4 

847 2766 7441 

Far beach 

Tapu 

SDV792 4.81% 569.6 0.6 

2 

422.0 229 3.3 171 623.7 1308 

Far beach 

Thornton 

SDV798 1.36% 1449 1.5 1309 1059 7.4 618 495.6 3853 

Far beach 

Kuranui Bay 

SDB616 2.35% 1026 1.3 97 753 3.4 366 1119 3345 

Far beach SDB559 87.2% 27 0.2 28.7 13.9 0.6 9.9 33.5 117 

1 

1 

SDB562 37.7% 210 1.6 48.3 61.3 3.9 19.9 74.8 480 

Depth samples SDB815 

(25- 

61.2% 244 3.4 26.8 72.9 8.4 12.1 99.2 766 

30cm) 

Te Puru 

Far beach 

Tararu Stream 

SDB608 2.1% 125 1.9 1157 1071 3.8 395 995 3510 

Near beach SDB602 86.6% 23.7 0.1 30.8 16.2 0.3 10.4 35.2 116 

Far beach 

Thames 

Harbour 

SDB598 2.99 866 3.3 512 860 5.0 194 816 3077 

SDB581 19.1% 40.8 0.2 65.2 42.9 0.5 22.3 41.6 173 

1 

0 

SDB584 9.49% 180 0.5 302 195 16. 90 130 605 

Thames 

stormwater 

pipeline 

Thames 

stormwater 

(near shore) 

Waihao River 

Inland River 

mouth 

SDB576 

A 

SDB565 

A 

SDB566 

A 

Doc # 1120743 Page 61 

4 

3 

0 

4.76% 309 1.1 588 349 4.2 186 336 1475 

94.6% 27.2 0.5 

6 

77.7% 557 0.4 

1 

SDB602 86.6% 23.7 0.1 

4 

SDB573 

B 

58.8% 18.9 0.1 

9 

SDB575 95.8% 9.8 0.1 

5 

9 

27.8 19.6 0.5 

7 

29.5 21.2 0.6 

8 

30.8 16.2 0.3 

9 

41.5 19.9 0.3 

9 

31.4 12.7 0.3 

0 

10.0 41.8 193 

10.9 51.5 187 

10.4 35.2 116 

14.3 45.1 142 

10.4 35.5 116

Location Sample % As Cd Cr Cu Hg Ni Pb Zn 

number Fines 

SDB579 93.7% 11.2 0.2 27.0 11.6 0.3 9.5 32.9 121 

9 

5 

SDB571 94.0% 10.2 0.2 27.8 11.6 0.3 9.5 33.9 114 

3 

6 

B. Grain size normalisation. Metal concentration divided by lithium 

concentration (normalised figures are unitless). 

Location 

Sample 

number 


Li 

(mg/kg) Metal Conc/Li 

Waiomu Bay 

Far beach SDB612 16.5 2.1 0.01 1.9 2.5 0.01 0.52 4.0 5.9 

Te Mata 

SDB614 15.2 1.9 0.01 1.8 2.3 0.01 0.62 2.0 5.4 

Far beach 

Tapu 

SDV792 17.1 1.6 0.002 1.2 0.64 0.01 0.48 1.8 3.7 

Far beach 

Thornton 

SDV798 15.7 1.3 0.001 1.1 0.92 0.01 0.54 0.43 3.3 

Far beach SDB616 20.3 1.2 0.001 1.1 0.86 0.004 0.42 1.3 3.9 

Kuranui Bay 

Far beach SDB559 28.6 0.82 0.01 0.87 0.42 0.02 0.30 1.0 3.6 

SDB562 19.5 4.1 0.03 0.93 1.2 0.07 0.38 1.4 9.3 

Depth 

samples 

SDB815 

(25-30cm) 17.0 8.8 0.12 0.96 2.6 0.30 0.44 3.6 27.6 

Te Puru 

Far beach SDB608 23.1 1.1 0.002 1.1 0.97 0.003 0.36 0.90 3.2 

Tararu Stream 

Near beach SDB602 31.4 0.65 0.004 0.85 0.45 0.01 0.29 0.97 3.2 

Far beach SDB598 17.7 1.5 0.01 0.86 1.5 0.01 0.33 1.38 5.2 

Thames Harbour 

SDB581 14.9 1.0 0.01 1.7 1.1 0.01 0.57 1.1 4.4 

SDB584 11.6 1.5 0.004 2.5 1.6 0.13 0.73 1.1 4.9 

QA/QC SDB576A 13.5 1.1 0.004 2.1 1.2 0.01 0.67 1.2 5.2 

Thames stormwater pipeline 

SDB565A 26.9 0.96 0.02 0.98 0.69 0.02 0.35 1.5 6.8 

SDB566A 22.6 19.2 0.01 1.0 0.73 0.02 0.38 1.8 6.4 

Thames stormwater (near shore) 

SDB602 31.4 0.65 0.004 0.85 0.45 0.01 0.29 0.97 3.2 

Waihao River (outer zone) 

OZ SDB573B 21.1 0.53 0.01 1.2 0.55 0.01 0.40 1.3 4.0 

Inland river mouth 

SDB575 30.8 0.31 0.005 0.98 0.34 0.01 0.32 1.1 3.6 

SDB579 22.2 0.47 0.01 1.1 0.49 0.01 0.40 1.4 5.1 

SDB571 25.8 0.37 0.01 1.0 0.42 0.01 0.34 1.2 4.1 

Page 62 Doc # 1120743

C. Grain size normalisation Metal concentration divided by iron concentration 

(normalised figures are unitless). 

Location 


Sample 

Number Fe (mg/kg) Metal Conc/Fe (x 10 -4 ) 

Waiomu Bay 

Far beach SDB612 44900 7.6 0.04 6.9 9.3 0.02 1.9 14.5 21.6 

Te Mata 

SDB614 35900 8.2 0.03 7.5 9.8 0.04 2.6 8.6 23.0 

Far beach 

Tapu 

SDV792 32300 8.5 0.01 6.3 3.4 0.05 2.5 9.3 19.5 

Far beach 

Thornton 

SDV798 25600 7.7 0.01 7.0 5.6 0.04 3.3 2.6 20.5 

Far beach 

Kuranui Bay 

SDB616 32400 7.4 0.01 7.0 5.4 0.02 2.7 8.1 24.3 

Far beach SDB559 30000 7.8 0.06 8.3 4.0 0.18 2.9 9.7 34.0 

SDB562 

SDB815 

32900 24.1 0.19 5.5 7.0 0.44 2.3 8.6 55.0 

Depth samples (25-30cm) 

Te Puru 

30300 49.2 0.68 5.4 14.7 1.7 2.4 20.0 154.8 

Far beach SDB608 38500 6.9 0.01 6.3 5.8 0.02 2.2 5.4 19.1 

Tararu Stream 

Near beach SDB602 37500 5.5 0.03 7.1 3.7 0.09 2.4 8.1 26.7 

Far beach SDB598 40800 6.3 0.02 3.8 6.3 0.04 1.4 6.0 22.5 


SDB581 34700 4.5 0.02 7.2 4.7 0.05 2.4 4.6 19.0 

SDB584 33200 5.2 0.02 8.6 5.6 0.46 2.6 3.7 17.3 

QA/QC SDB576A 34100 4.3 0.01 8.2 4.9 0.06 2.6 4.7 20.6 


SDB565A 31900 8.1 0.17 8.2 5.8 0.17 3.0 12.4 57.4 

SDB566A 53400 81.1 0.06 4.3 3.1 0.10 1.6 7.5 27.2 


SDB602 


37500 5.5 0.03 7.1 3.7 0.09 2.4 8.1 26.7 

OZ SDB573B 29800 3.7 0.04 8.2 3.9 0.08 2.8 8.9 28.1 


SDB575 33500 2.8 0.04 9.0 3.6 0.09 3.0 10.1 33.1 

SDB579 24800 4.2 0.11 10.2 4.4 0.13 3.6 12.4 45.6 

SDB571 27500 3.5 0.08 9.5 4.0 0.12 3.2 11.6 38.9 

Doc # 1120743 Page 63

D. Grain size normalisation. Metal concentration divided by aluminium 

concentration (normalised figures are unitless). 

Location 

Sample 

Number 


Al 

mg/kg Metal Conc/Al (x 10 -4 ) 

Waiomu Bay 

Far beach SDB612 8830 35.5 0.19 35.0 47.1 0.10 9.7 74.0 110.0 

Te Mata 

SDB614 8890 33.3 0.11 30.3 40.0 0.18 10.6 34.5 92.9 

Far beach 

Tapu 

SDV792 18300 15.0 0.02 11.1 6.0 0.09 4.5 16.4 34.4 

Far beach 

Thornton 

SDV798 12900 15.3 0.02 13.8 11.2 0.08 6.5 5.2 40.6 

Far beach 

Kuranui Bay 

SDB616 11000 21.9 0.03 20.7 15.8 0.07 7.8 23.9 71.5 

Far beach SDB559 23300 10.0 0.08 10.7 5.2 0.23 3.7 12.5 43.8 

SDB562 

SDB815 

13500 58.7 0.45 13.5 17.1 1.1 5.6 20.9 134.1 

Depth samples 

Te Puru 

(25-30cm) 12300 121.1 1.7 13.3 36.3 4.2 6.0 49.3 381.3 

Far beach 

Tararu Stream 

SDB608 13100 20.2 0.03 18.5 17.2 0.06 6.3 16.0 56.3 

Near beach SDB602 23400 8.8 0.05 11.4 6.0 0.15 3.8 13.0 42.7 

Far beach SDB598 12500 20.7 0.08 12.2 20.6 0.12 4.6 19.5 73.6 


SDB581 20500 7.6 0.04 12.1 8.0 0.09 4.1 7.8 32.1 

SDB584 16600 10.3 0.03 17.3 11.1 0.92 5.1 7.4 34.6 

QA/QC SDB576A 15300 9.6 0.03 18.3 10.8 0.13 5.9 10.5 45.9 


SDB565A 23300 11.0 0.23 11.3 7.9 0.23 4.1 17.0 78.5 

SDB566A 18900 229.1 0.17 12.1 8.7 0.28 4.5 21.2 76.7 


SDB602 


23400 8.8 0.05 11.4 6.0 0.15 3.8 13.0 42.7 

OZ SDB573B 17000 6.5 0.06 14.4 6.9 0.14 4.9 15.6 49.2 


SDB575 26500 3.5 0.05 11.4 4.6 0.11 3.8 12.8 41.9 

SDB579 18800 5.6 0.14 13.5 5.8 0.18 4.7 16.4 60.1 

SDB571 19600 4.9 0.11 13.3 5.6 0.17 4.5 16.3 54.6 

Page 64 Doc # 1120743

Appendix 5. Statistical summary of results for eight trace 

elements in each of the ten areas sampled. 

All results are mg/kg dry wt. Individual sample results highlighted in yellow exceed the 

ANZECC (2000) ISQG-Low; those in red exceed the ANZECC ISQG-High (refer to 

Section 2.5). 

Waiomu Bay 


SDB611 34.5 0.1 26.1 39.9 0.08 8.9 45 79.2 

SDB613 26.3 0.12 30 34.1 0.06 9.8 37.5 83.3 

SDB612 34 0.17 30.9 41.6 0.09 8.6 65.3 97.1 

SDB614 29.6 0.1 26.9 35.2 0.16 9.4 30.7 82.6 

SDB 615 21 0.02 20.8 17 0.07 7.9 20.9 71.7 

SDB 594 25.9 0.1 29.2 39.4 0.07 9.8 29.7 84.1 

SDB619A 21 0.05 22 21 0.05 8 26.5 79 

Maximum 21 0.02 20.8 17 0.05 7.9 20.9 71.7 

Minimum 34 0.17 30.9 41.6 0.16 9.8 65.3 97.1 

Median 26.3 0.10 26.9 35.2 0.07 8.9 30.7 82.6 

Average 27.5 0.09 26.6 32.6 0.1 8.9 36.5 82.4 

Std dev 5.5 0.05 3.9 9.7 0.0 0.8 14.9 7.7 

95% conf error 6.9 0.06 4.9 12.1 0.0 1.0 18.5 9.6 


Guideline value 20 1.5 80 65 0.15 21 50 200 

Definitely below TV 15.0 0.5 65.4 40.9 0.1 18.7 24.7 176.8 

Definitely above GL 25.0 2.5 94.6 89.1 0.2 23.3 75.3 223.2 

95% lower 20.6 0.0 21.7 20.5 0.0 7.9 18.1 72.9 

95% upper 34.3 0.2 31.4 44.7 0.1 9.9 55.0 92.0 

Te Mata 


SDV790 23.4 0.03 19.9 11.4 0.16 8.4 25.9 60.4 

SDV795 27.3 0.03 21.5 11.3 0.15 9.1 30.6 64.4 

SDV794 27.4 0.03 20.3 11 0.16 8.2 30 62.9 

SDV791 29.6 0.1 26.9 35.2 0.16 9.4 30.7 82.6 

SDV792 27.4 0.03 20 11.2 0.15 8.4 31.5 64.1 

SDV793 21 0.02 20.8 17 0.07 7.9 20.9 71.7 

SDB 593 25.9 0.1 29.2 39.4 0.07 9.8 29.7 84.1 

Minimum 21 0.02 19.9 11 0.07 7.9 20.9 60.4 

Maximum 29.6 0.1 29.2 39.4 0.16 9.8 31.5 84.1 

Median 27.3 0.03 20.8 11.4 0.15 8.4 30 64.4 

Average 26.0 0.05 22.7 19.5 0.1 8.7 28.5 70.0 

Std dev 2.9 0.04 3.8 12.4 0.0 0.7 3.8 9.7 

95% conf error 3.6 0.04 4.7 15.4 0.1 0.9 4.7 12.1 


Guideline value 20 1.5 80 65 0 21 50 200 

Definitely below GL 17.2 0.1 63.4 13.7 0.1 18.9 41.7 165.5 


95% lower 22.4 0.00 18.0 4.1 0.1 7.9 23.8 57.9 

95% upper 29.6 0.09 27.4 34.9 0.2 9.6 33.2 82.1 

Doc # 1120743 Page 65

Tapu 


SDV796 23.4 0.02 22.7 13.9 0.11 8.7 9.32 58.2 

SDV801 18.8 0.02 19.2 13.7 0.09 8.1 8.16 53.1 

SDV800 16.9 0.03 19.5 13.7 0.13 8.4 11.1 57.4 

SDV797 27.3 0.02 18.3 14 0.21 8.2 5.63 51.5 

SDV798 19.7 0.02 17.8 14.4 0.1 8.4 6.74 52.4 

SDV799 21 0.02 19.8 14.2 0.1 8.2 7.93 53.7 

Minimum 27.3 0.03 22.7 14.4 0.21 8.7 11.1 58.2 

Maximum 16.9 0.02 17.8 13.7 0.09 8.1 5.63 51.5 

Median 20.35 0.02 19.4 14 0.11 8.3 8 53.4 

Average 21.2 0.02 19.6 14.0 0.1 8.3 8.1 54.4 

Std dev 3.7 0.004 1.7 0.3 0.04 0.2 1.9 2.8 

95% conf error 4.6 0.01 2.1 0.3 0.1 0.3 2.4 3.4 





95% lower 16.6 0.0 17.4 13.6 0.1 8.1 5.8 51.0 

95% upper 25.8 0.0 21.7 14.3 0.2 8.6 10.5 57.8 

Thornton 


SDV802 21.9 0.03 20.6 17.2 0.07 8.2 22.7 72.1 

SDV803 24.4 0.03 22 19.3 0.08 7.8 20 70.2 

SDV804 24 0.03 20.6 18.3 0.07 8 19.6 69.4 

SDB615 21 0.02 20.8 17 0.07 7.9 20.9 71.7 

SDB616 24.1 0.03 22.8 17.4 0.08 8.6 26.3 78.6 

SDB621 22.7 0.04 22.1 17.6 0.09 8.3 23.6 74 

Minimum 21 0.02 20.6 17 0.07 7.8 19.6 69.4 

Maximum 24.4 0.04 22.8 19.3 0.09 8.6 26.3 78.6 

Median 23.35 0.03 21.4 17.5 0.075 8.1 21.8 71.9 

Average 23.0 0.03 21.5 17.8 0.1 8.1 22.2 72.7 

Std dev 1.6 0.01 0.7 1.1 0.0 0.2 1.4 1.3 

95% conf error 2.0 0.01 0.8 1.3 0.0 0.2 1.7 1.6 





95% lower 21.0 0.02 20.6 16.5 0.1 7.9 20.5 71.1 

95% upper 25.0 0.04 22.3 19.1 0.1 8.3 23.9 74.2 

Page 66 Doc # 1120743

Kuranui Bay 


SDB563 20.7 0.34 22.4 15.8 0.36 7.6 28.7 130 

SDB560 20.7 0.34 22.4 15.8 0.36 7.6 28.7 130 

SDB561 73.7 0.69 16.1 22.3 1.43 6.8 30 206 

SDB559 23.4 0.18 25 12.1 0.53 8.6 29.2 102 

SDB562 79.3 0.61 18.2 23.1 1.46 7.5 28.2 181 

SDB591 23.4 0.18 25 12.1 0.53 8.6 29.2 102 

SDB812 16.8 0.19 29 12.9 0.35 22.4 33.9 116 

EW KB 19.4 0.34 15.7 21.2 0.84 6.4 42.9 199 

Minimum 16.8 0.18 15.7 12.1 0.35 6.4 28.2 102 

Maximum 79.3 0.69 29 23.1 1.46 22.4 42.9 206 

Median 22.05 0.34 15.8 22.4 0.53 7.6 29.2 130 

Average 34.7 0.4 21.7 16.9 0.73 9.4 31.4 145.8 

Std dev 25.9 0.2 4.7 4.6 0.5 5.3 5.0 43.0 

95% conf error 32.2 0.24 5.9 5.8 0.6 6.6 6.2 53.3 





95% lower 2.5 0.1 15.9 11.2 0.2 2.9 25.1 92.4 

95% upper 66.9 0.6 27.6 22.7 1.3 16.0 37.6 199.1 

Te Puru 


SDB604 22.4 0.03 22.4 20.4 0.06 8.2 19.2 69.2 

SDB603 25 0.18 12.9 21.9 0.15 5.3 19.2 81.2 

SDB606 26.4 0.04 24.3 22.5 0.08 8.3 20.9 73.7 

SDB609 26.5 0.04 24.6 26.5 0.09 8.6 21.1 76.4 

SDB608 26.4 0.04 24.3 22.5 0.08 8.3 20.9 73.7 

SDB607 22.8 0.03 22.5 20 0.06 8.3 18.6 68.3 

SDB816 21.6 0.03 20.9 19.4 0.13 8 20 65.2 

Minimum 21.6 0.03 12.9 19.4 0.06 5.3 18.6 65.2 

Maximum 26.5 0.18 24.6 26.5 0.15 8.6 21.1 81.2 

Median 25 0.04 22.5 21.9 0.08 8.3 20 73.7 

Average 24.4 0.06 21.7 21.9 0.09 7.9 20.0 72.5 

Std dev 2.1 0.06 4.1 2.4 0.03 1.1 1.0 5.4 

95% conf error 2.6 0.07 5.1 3.0 0.04 1.4 1.2 6.7 



Definitely below GL 17.8 -0.3 61.2 56.2 0.1 17.2 46.9 181.5 


95% lower 21.8 -0.01 16.6 18.9 0.1 6.4 18.7 65.8 

95% upper 27.1 0.12 26.8 24.8 0.1 9.3 21.2 79.2 

Doc # 1120743 Page 67

Tararu Stream As Cd Cr Cu Hg Ni Pb Zn 

SDB602 20.5 0.12 26.7 14 0.34 9 30.5 100 

SDB601 23.1 0.18 14.2 23.3 0.15 5 24.5 93.8 

SDB600 30.4 0.07 13.9 23.8 0.15 5.5 25.4 87.6 

SDB597 21.1 0.16 13.5 20.9 0.18 5.7 25.5 80.3 

SDB598 25.9 0.1 15.3 25.7 0.15 5.8 24.4 92 

SDB599 24.4 0.32 14.8 22.3 0.14 5.7 24.8 136 

SDB 595 26.3 0.21 15.8 31.3 0.14 5.6 24.1 104 

Minimum 20.5 0.07 13.5 14 0.14 5 24.1 80.3 

Maximum 30.4 0.32 26.7 31.3 0.34 9 30.5 136 

Median 24.4 0.16 14.8 23.3 0.15 5.7 24.8 93.8 

Average 24.5 0.17 16.3 23.0 0.18 6.0 25.6 99.1 

Std dev 3.4 0.08 4.6 5.2 0.07 1.3 2.2 18.0 

95% conf error 4.2 0.10 5.8 6.5 0.09 1.7 2.8 22.4 





95% lower 20.3 0.06 10.5 16.6 0.1 4.4 22.8 76.7 

95% upper 28.8 0.27 22.1 29.5 0.3 7.7 28.4 121.5 

Waihou River 


SDB573B 11.1 0.11 24.4 11.7 0.23 8.4 26.5 83.7 

SDD573E 13 0.12 20.1 8.4 0.2 7.5 23.2 74 

SDB574 5.1 0.08 31.4 7.6 0.14 8.4 19 72.3 

SDB575 9.4 0.14 30.1 12.2 0.29 10 34 111 

SDB568 7.5 0.17 25.1 9.7 0.27 8.4 28.9 98.5 

SDB569 9.6 0.23 26.4 11.1 0.34 9 32.5 113 

SDB578 7.5 0.19 21.5 9.3 0.28 7.6 25.7 90.9 

SDB579 10.5 0.27 25.3 10.9 0.33 8.9 30.8 113 

SDB571 9.6 0.22 26.1 10.9 0.34 8.9 31.9 107 

SDB580 14.2 0.04 26.1 16 0.12 8.7 14 61.1 

SDB590 14.1 0.05 27.7 16.3 0.19 8.6 14.5 63.1 

Minimum 5.1 0.04 20.1 7.6 0.12 7.5 14 61.1 

Maximum 14.2 0.27 31.4 16.3 0.34 10 34 113 

Median 9.6 0.14 26.1 10.9 0.27 8.6 26.5 90.9 

Average 10.1 0.15 25.8 11.3 0.25 8.6 25.5 89.8 

Std dev 2.9 0.08 3.3 2.8 0.08 0.7 7.1 20.1 

95% conf error 3.6 0.09 4.1 3.4 0.10 0.8 8.8 24.9 





95% lower 6.6 0.05 21.8 7.8 0.15 7.7 16.7 64.9 

95% upper 13.7 0.24 29.9 14.7 0.35 9.4 34.4 114.7 

Page 68 Doc # 1120743



SDB581 15.6 0.08 24.9 16.4 0.19 8.5 15.9 65.9 

SDB582 13.8 0.03 21.9 13.5 0.14 8.8 15.5 56.2 

SDB583 15.1 0.05 23.6 14.2 0.17 8.6 17 59.7 

SDB584 17.1 0.05 28.7 18.5 1.52 8.5 12.3 57.4 

SDB585 14.3 0.06 21.7 14.1 0.15 8.4 16.1 59.7 

SDB576A 14.7 0.05 28 16.6 0.2 9 16 70.2 

SDB576B 16.3 0.04 21.7 14.6 0.17 7.2 12.5 52.5 

SDB576C 17.8 0.04 30.7 17 1.58 8.8 15.3 64.3 

SDB576D 15.8 0.04 24.4 16.1 0.2 8.5 13.2 55.8 

SDB576E 12.7 0.07 30.3 16 0.22 9.4 17.5 72.9 

SDB567 8.7 0.1 22.8 10 0.21 7.3 25 80 

SDB602 20.5 0.12 26.7 14 0.34 9 30.5 100 

Thames GC 13.2 0.05 16.5 10 0.22 8.1 28.9 62.2 

Minimum 8.7 0.03 16.5 10 0.14 7.2 12.3 52.5 

Maximum 20.5 0.12 30.7 18.5 1.58 9.4 30.5 100 

Median 15.1 0.05 24.4 14.6 0.2 8.5 16 62.2 

Average 15.0 0.06 24.8 14.7 0.4 8.5 18.1 65.9 

Std dev 2.8 0.03 4.0 2.5 0.5 0.6 6.0 12.8 

95% conf error 3.5 0.03 5.0 3.1 0.63 0.8 7.5 15.9 



Definitely below GL 15.3 0.69 63.8 51.1 -0.08 19.1 29.4 151.7 




SDB564 27.1 0.31 26.5 15.8 0.42 9.3 37.1 143 

SDB565 25.7 0.53 26.3 18.5 0.54 9.5 39.5 183 

SDB566 433 0.32 22.9 16.5 0.53 8.5 40 145 

SDB810 16.3 0.32 25 14.3 0.37 8.7 30.7 124 

Minimum 16.3 0.31 22.9 14.3 0.37 8.5 30.7 124 

Maximum 433 0.53 26.5 18.5 0.54 9.5 40 183 

Median 26.4 0.32 25.65 16.15 0.48 9 38.3 144 

Average 125.5 0.37 25.2 16.3 0.47 9.0 36.8 148.8 

Std dev 205.0 0.11 1.7 1.7 0.08 0.5 4.3 24.7 

95% conf error 254.5 0.13 2.1 2.2 0.10 0.6 5.3 30.7 



Definitely below GL -20.6 1.0 73.5 56.4 0.1 19.6 42.8 158.7 


95% lower -129.0 0.24 23.1 14.1 0.4 8.4 31.5 118.1 

95% upper 380.1 0.50 27.2 18.4 0.6 9.6 42.1 179.4 

Doc # 1120743 Page 69

Appendix 6. Concentrations of 33 elements, total organic 

carbon and dry matter in composite shallow (0-2 cm) sediment 

samples collected in October 2003 from five locations in the 

Firth of Thames and five locations in Raglan Harbour. 

All trace element concentrations are in units of mg/kg (dry weight). In all cases but the 

first Te Puru entry, composites comprised five equal samples from each location, with 

each sample comprising twelve subsamples. In the case of first Te Puru entry, samples 

that made up the composite were analysed individually, and the reported result for 

each element is the average of the five results. Results for the Te Puru composite 

sample compare well with these averages. This is a means of validating the composite 

analysis approach. 

Constituent 

Te 

Puru 

Te 

Puru 

Firth of Thames sites . Raglan Harbour sites 

Kaiaua Miranda Kuranui 

Bay 

Thames 

Ponganui 

Creek 

Whatitirinui 

Is 

Calcium 20960 22417 12100 27500 18600 10100 16300 26000 39200 13300 8980 

Magnesium 5900 6147 3220 3390 5930 5560 2710 4040 3080 4130 4930 

Sodium 4710 4938 3990 4210 5750 7920 5640 5930 5720 7070 8750 

Potassium 908 934 1030 949 1110 1760 1150 1370 1090 1400 1970 

Lithium 17.0 17.6 11.3 10.4 18.7 10.9 8.9 13.6 9.9 15.6 19.5 

Rubidium 3.20 3.28 7.55 6.54 4.57 4.21 6.29 6.65 5.85 7.9 9.95 

Phosphorus 376 385 212 215 364 343 347 552 554 649 543 

Boron 8.60 8.83 11 10 13 12 15 16 14 18 22 

Iron 32600 33033 15000 11200 29200 27200 13700 25300 23400 24900 28200 

Manganese 549 567 327 272 1480 747 131 299 261 300 297 

Silver 0.076 0.092 0.05 0.05 0.3 0.09 0.02 < 0.02 0.15 0.03 0.03 

Aluminium 10140 10617 6470 4880 9300 14400 6020 10600 8180 12600 14300 

Arsenic 16.84 17.3 5.3 4.3 19.4 13.2 6.9 7.7 7.2 9 10.9 

Barium 4.82 4.91 6.76 5.16 4.94 9.12 7.66 12.8 11.6 14.2 18 

Bismuth 0.294 0.285 0.07 0.04 0.3 0.13 0.04 0.05 0.04 0.07 0.1 

Cadmium 0.04 0.04 0.02 0.03 0.34 0.05 0.03 0.02 0.02 0.02 0.03 

Cobalt 11.3 11.7 9.11 5.85 12.2 11.2 3.85 8.23 7.74 8.27 8.98 

Chromium 21.3 21.9 7.1 7.1 15.7 16.5 10.5 14.5 13.2 16.8 17.2 

Caesium 0.752 0.763 0.92 0.74 1.2 0.53 0.59 0.73 0.61 0.9 1.17 

Copper 17.5 17.6 5.4 3.7 21.2 10 3.3 7 6.1 8.4 9 

Mercury 0.472 0.103 0.05 0.05 0.84 0.22 0.02 0.02 0.02 0.03 0.04 

Lanthanum 4.86 5.12 10 9.26 5.98 7.85 5.74 8.4 9.39 9.16 12.9 

Molybdenum 0.906 0.925 0.2 0.18 1.22 0.59 0.34 0.3 0.36 0.37 0.42 

Nickel 7.30 7.48 4.2 3.9 6.4 8.1 4.9 8.4 6.9 9.8 10.3 

Lead 23.3 23.9 15.3 11.5 42.9 28.9 4.34 5.6 4.6 6.73 9.19 

Antimony 0.590 0.602 0.15 0.04 0.81 0.28 0.11 0.12 0.12 0.12 0.16 

Selenium

Appendix 7. Results of Student’s t-tests between Thames 

and Kuranui Bay sites and three mining reference sites 

(Te Puru, Thornton and Te Mata). 

Mean values are in mg/kg (dry weight) and have not been normalised to any other 

variable. Ratios of means are unitless. 

Mean of control sites (Te 

Puru, Thornton and Te Mata) 

(N=20) 


24.7 0.040 21.3 17.3 0.105 8.16 23.8 69.7 

Mean at Kuranui Bay (N=9) 


36.1 0.407 22.71 16.37 0.763 8.1 29.92 145.8 


control site 

No No No No No No No No 



significant 

1.5 10.3 - - 7.3 - 1.3 2.1 

Mean at Thames stormwater 

pipeline (N=5, except N=4 for 

arsenic due to one outlier 

being removed) 


24.4 0.396 23.9 16.9 0.47 8.74 35.6 149 


control site 

No Yes Yes No Yes No Yes Yes 



significant 

- 10.0 1.1 - 4.5 - 1.5 2.1 

Mean at Thames mudflats, 

deep harbour, wharf and 

stormwater sites (N=11) 


14.2 0.084 24.81 14.01 0.355 8.48 19.89 72.36 


control site 

No Yes Yes No Yes No No No 



significant 

- 2.1 1.2 - 3.4 - - - 

Doc # 1120743 Page 71

Appendix 8. Map showing the full extent of catchments 

feeding the lower Firth of Thames. 

For closer detail of the Firth of Thames area, refer to Figures 2-1 and 2-2. 

Page 72 Doc # 1120743

Trace Elements in Sediments of the Lower Eastern Coast of the Firth ...

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